When you remove your old bathtub or shower, you may be surprised to find that there's nothing behind that fiberglass wall, but maybe some studs and insulation. You may even be surprised to find some mold or other moisture related problems that you'd hoped not to find.
Installing "Green Board"
Green Board is a moisture resistant drywall that can be cut to size and nailed into place behind your future walk-in tub surround. You'll not need to go all the way to the floor, leaving roughly 18" of a gap is fine for this type of installation (as it will be hidden by the tub).
It's a good idea to caulk the corners and any gaps you may find above your 18" installation height to prevent moisture or humidity from trapping itself behind your new wall (even though you'll have a surround over top of it).
Covering things up
If you had a shower head previously, this is the perfect opportunity to hide the old plumbing and not have a useless shower head sticking out of your wall. Likewise, if there were any other dated or unsightly cosmetic problems, your new wall and surround will take care of these.
Adding a new surround is the perfect time to ask yourself about storage options. What do you need, and what would make bathing easier in your bathroom? Were you lacking storage for soaps or essential oils before? Consider purchasing a surround that has recessed storage spaces, but be sure they're higher than the highest section of the walk-in tub.