Car or Train to visit cities in Bavaria (Except Munich)

Every place you mentioned is readily accessible by rail. Some might take a little less time by car, but, IMO, not enough less to warrant the higher cost and higher inconvenience (e.g., parking) with a car.

Almost all Cities you plan to visit have a direct train connection from/to Munich
Except Rothenburg ob der Tauber with 2 changes in Würzburg and Steinach

So when not planning to go to rural areas in Bavaria from my point of view no need to rent a car otherwise I would

Have a look at the regional train network of Bavaria.

If you want to travel a lot on one day in regional trains of Bavaria (Bayern), check out Bayern-Ticket.

to visit all the cities in Bavaria

Good you mentioned the ones you mean because Bavaria has 318 towns.

She told me Munich is the best and just ride a train to visit the
towns like Würzburg, Nuremberg, Rosenheim, etc. I'm planning to
visit Regensburg, Würzburg, Rothenburg ob der Tauber, Bamberg,
Rosenheim, Füssen.

You can do all these places by train without the hassle, expense, and risk that accompanies the rental car option. Her advice works for some of your destinations, not for others...

TICKETS: Let's say you are traveling all 6 of those days as stated. The Deutschland Ticket (€58/mo) will be more flexible and cheaper than any combination of day pass options (Bayern Ticket, etc.)

https://int.bahn.de/en/offers/regional/deutschland-ticket
https://www.seat61.com/train-travel-in-germany.htm#Deutschlandticket

The cities I highlighted above are TOO FAR from Munich to be considered day trips unless you want to start paying heavier prices for ICE trains. Würzburg is 4 hours round trip even with those! And 6 hours by regional train. So IMHO thee smart move is to transfer your base town for 2-3 days to NUREMBERG.

You can get to Nuremberg by regional train one way in 1.7 hours. BAMBERG is 1.5 hrs. round trip. From NUREMBERG, Würzburg takes about 2.2 hours round trip, and Rothenburg about 3.5 hours, round trip (which is a little longer but doable since it's a small town, and much more doable than from Munich - which would take 7-8 flippin' hours.)

she was right

Nuremberg makes a good base for travel to Wuerzburg, Rothenburg o.d.T., Regensburg, and Bamberg. Shorter distances and travel time. Bamberg and Rothenburg are in the VGN area with cheaper and more flexible day tickets.

Great advice from your friend and fellow travelers.

Additionally, I find that train time is great for a last minute review of my touring plans for my destination. I will have guide books, maps, etc. previously downloaded to my phone and I do a final check enroute. At the end of the day the train ride home is a great way to decompress, make travel notes and post pictures or videos on social media. And, if I have dinner at my destination for the day, I don't have to worry about driving after a glass of wine or beer.

If all you want to see are the main cities then take the train and use the public transportation there. If you want to get out and about rent the car. Parking in bigger cities can be a real problem. You can always break away for a few nights with a rental and see other things. A road trip with different overnights. Plan your trip for sights/interests and where you will stay and want you want to to eat. There are a lot of in between places that public transport covers in Germany, but it takes a long time and limited routes and time tables. Some places a car is the only thing practical with limited time.

Except Rothenburg ob der Tauber with 2 changes in Würzburg and Steinach

Actually, I see regional trains (RE 80 & 89), from Munich to Würzburg, stopping on the way in Steinach, so only one change (and there are no direct trains into Rothenburg from anywhere but Steinach. You will always have that change.)

Some places a car is the only thing practical

Actually, in over 160 days of travel in Germany, I've never found any place, to which I wanted to go, that wasn't accessible by rail or by bus from a nearby rail station.

Some places a car is the only thing practical with limited time.

I would suggest getting into the mountains at least once. Take a lift up. There is nothing like sitting in the mountains with sunshine, a view and a Bier. A little Volksmusik aus Bayern, Österreich und den Alpen is nice. Prost