Probably because RER Line B doesn't actually run direct to Montparnasse - going this way requires a change at one of several inner-city Metro stations onto a line that does go through Montparnasse. So there's that - and from my experience the distance between the Metro station part of Montparnasse (M. Bienvenue) and the main Gare Montparnasse is the longest I've ever had to walk within one (technically) station! Actually they are really two separate stations joined by underground passages. They do have a moving walkway at one point I think but boy, it felt like a hike when I was trying to get to my train (admittedly with a suitcase, this wasn't on a Camino trip). I wouldn't personally use that method of getting from one to the other, although if you like the Metro, feel like stretching your legs a bit, have plenty of time and very little in the way of baggage (the physical variety anyway!), it would be fine. Ditto if you want to get off earlier and spend a bit of time walking the streets of Paris. Compared to a door-to-door bus though, I would go with the bus. The time involved wouldn't be that much more once you factor in the
correspondances and potentially rush-hour Metro crowds. Or fly, of course!
To answer your question about booking - you don't need to (and can't) book ahead for the RER or Metro trains, but they run frequently. For the TGV from Montparnasse to Bayonne (or from CDG to Bayonne with a change at Bordeaux) I always use the official SNCF site (French rail company) at
https://www.oui.sncf/. others prefer one of the many third party sites such as Trainline. Raileurope mentioned above is connected to SNCF and you may be redirected there if you are booking from outside Europe (there are ways around this if you prefer to book direct).