Advance-sale, "saver fare" or "supersaver fare" tickets - which are for journeys that include at least one long-distance (IC, ICE, EC for example) train - are capacity-controlled for each specific train. As tickets sell, the price per ticket goes higher and higher. Thus a June 1 ticket for a 9:00 train from Frankfurt to Munich that is sold on January 1 might sell for €29, but if bought on May 1, it is very likely to sell for 2 or 3 times that amount... in any case, something closer to the normal price (€111) of the "flex-fare" ticket (which is not train-specific - you can travel the same route on any train that day - and also is fully refundable/flexible.)
Flex-fare (normal fare) tickets can be bought in advance, but you don't save anything. So the main point of advance-sale tickets is to clear some savings.
There are no "saver fare" tickets for regional-train-only journeys or other journeys that do not include long-distance trains. So in the case of Boppard > Frankfurt, you'd have a choice; some journeys include a change of train from regional to long-distance train. For example, on June 1, you can buy a saver fare ticket today for the 18:12 - 20:09 Boppard > Frankfurt journey (€19.90.) But if you want a DIRECT train and specify such, there's no saver-fare option because there are no direct high-speed trains. Only the RE trains do that. And you will pay €26 (normal fare - the only fare, really) for a DB ticket no matter whether it's bought today or on June 1.
(There's a workaround to save €1.50 off the €26 fare by buying a RMV ticket + a DB ticket, but I'm not going to complicate this discussion for that tiny amount.)
Now, keep in mind that if you are staying in Boppard and if your guest ticket is valid on your day of departure (I'm not sure about that) then you don't need any ticket at all to ride the train from Boppard to Oberwesel. You need only a ticket for Oberwesel > Frankfurt (€21.80) on that RE train you plan to board in Boppard.
(Tip for those heading to FRA airport from Boppard or other Rhine towns: always do as Pat has done if you can - use a DIRECT train to minimize the chance that you'll arrive late at the airport!)
Normal regional train tickets are flexible, just like the flex-fare "normal price" tickets for long-distance trains. Take the Oberwesel > Frankfurt ticket we just discussed. You pay the €21.80 and you can travel at any hour that same day. You can also break up your journey if you like for a stopover in Bacharach or Mainz if you wish.
As for savings on regional-trains-only journeys you have to look for DAY PASSES... or more rarely MULTI_DAY PASSES... Either the Länder (state) passes or passes issued by local transit authorities (like the VRM, RMV, etc.) These typically offer some value for the single traveler and excellent value for 2 or more individuals traveling together and normally have an after-9-am-only restriction. The exact rules vary by state / local authority. You just buy them as you go at the station, as a rule. S-Bahn trains are included with the regional-trains-only day passes; other varieties of "Nahverkehr" transport (tram, bus, etc.) are often accessible as well with these day passes.