Train travel

I always reserve either directly with the train company (Trenitalia, SBB, OBB, SNCF, etc) or with www.trainline.eu as there are no extra fees. I do not understand why RS promotes Raileurope.

Book direct with the rail service you're using, there's no reason to use Raileurope. I think I used it once for Italy but it's preferable to book directly with the train service for the company.

Seconded
First choice buy from the company running the train. But some company websites are easier to use than others.
Second choice https://www.trainline.eu/ or https://loco2.com/ who are both European based agencies who do not give mark-ups. In both cases they only cover some countries, there are notable holes in their coverage.

Raileurope is a North-American based agency designed for North-American users: times in am/pm, back-to-front US style dates, English place names instead of the real names, and prices in USD. That makes it "accessible" for first timers without having to work out what company is running a train.
But I have seen too many cases of only some trains being listed, and prices higher than the rail company charges, so avoid.

I always reserve either directly with the train company (Trenitalia, SBB, OBB, SNCF, etc) or with www.trainline.eu

Or, if possible, with the German Rail (Bahn) website. Trainline has the same prices as the Bahn for most fares, but is sometimes misleading as to local fares. You have to check with the Bahn to be sure you have the best price, so check there first.

And never buy local and regional train tickets or regional passes in advance from Trainline or the Bahn. Once you buy them there can be penalties for changing your tickets. Buy them at the station at the time of travel for the best price.

I guess Rick Steves recommends Raileurope because he has a business relationship with them. Head and Shoulders Shampoo also recommends that I wash, rinse, and repeat.

Your question is too general, since different countries, types of train, and types of ticket have different rules, discounts, and online availability.

Your previous (also very cryptic) question mentions two weeks in France, over too wide a territory, with some destinations where driving would be more convenient. Have you now worked out the plan to the point where you're ready to book train tickets? If you/'re planning a trip for this spring, you should read about the ongoing French railway strike plan.

Rail Europe is an authorized agent that sells French rail tickets at similar prices and advance discounts as does SNCF, but not all of them, such as Ouigo. If you wanted a Eurail France Pass plus seat reservations, then Rail Europe sells those and the SNCF site does not.

I've worked very hard over the last 15-20 years to find the least expensive ways to travel in Europe (or at least Germany) - how to find the least expensive accommodations and book them yourself, using public transportation (not a car) and getting the best fares (not rail passes), eating local specialties to keep meal expenses down, and avoiding expensive 3rd party tours that just add cost without providing significant benefits.

A lot of the lower cost options require some work. Always out there, circling like vultures, are the people who hope to make money off of tourist by providing them with convenience in exchange for money, lots of money. I include in this category companies like TripAdvisor, Booking dot com (and similar booking websites), RailEurope, and 3rd party tour companies like Viator (what did PT Barnum say?). If you have lots of money you don't know what to do with, and you don't like figuring things out for yourself, go ahead and use them, but you are spending a lot of money uncessarily to take the easy way out.

Laura has given you the company line about RailEurope. My own, much more modest, experience suggests it offers a sometimes less-than-complete set of choices at the same or higher prices. I prefer to go directly to the source, the operating rail line, just as I find airlines are best place to shop for flights, at least on simple itineraries. I grant there may be exceptions, in both forms of transport I mention, so double-check before purchase. Also consider what the master of train travel has to say: www.seat61.com

I typically book directly with the rail company.

Adding to what Southam said, in October I went from Lindau to Rothenburg. For comparison, I picked a day next May, to get a good Savings Fare from the Bahn. For that date, RailEurope shows only one rail connection all day, a connection using an IC, leaving at ~9:30 (the time I actually wanted) and getting to Rothenburg at 14:30. The fare from RailEurope for a train specific fare with limited exchangeability would be $97 for 2. The Bahn showed that connection plus three other Savings Fare connections between 9 AM and noon, all for 59,80€ ($74) for 2. I actually took an all regional connection, leaving Lindau on the same 9:30 train with a Bayern-Ticket for 31€ (~$38). There were two all-regional connections for which the Bayern-Ticket was valid leaving Lindau by noon. Neither was shown on RailEurope.

The all-regional connection took an hour longer but saved me almost $60!

In Lee's example, I would note that the 9:20 departure is the only one of the day that makes the trip with only 2 connections. Other departures suggested by DB require 3 to 5 transfers. The limitation of RE's shopping cart to 2 connections on a ticket may be a system limitation or may be based on what they expect users to prefer; I don't know. Booking a long trip in shorter sections is the usual work-around.

Of course, we also encourage readers to use the national railway web sites. They're listed in every guidebook and linked on many pages of our rail travel info here. No single web site provides one-stop shopping nor does everything equally well.

You need to understand what RailEurope is. It is a travel agency to sell rail tickets to North Americans. They provide a lot of hand holding that some need, get a fee for doing that, and provide a service. There is nothing bad about them. Their service is very good. Some limitations are that they do not always list all of the trains available and sometimes they are not the lowest price. But for some, they provide a good level of service. So use them if you want to.