Biomes'O'Plenty is a world generation mod that when selected changes the terrain, and adds many new biomes to the game (not all are set to be used, so for most variety, check the configuration file)
If you use BoP for world generation, it will spawn the new biomes as well as the vanilla ones and doesn't affect generation of structures (unless you consider, for example, less desert biomes due to other biomes being generated = less desert temples)
If you use the regular world generation, no BoP biomes will be created at all (unless there is a bug... this was the case a few iterations ago)
The reason why the packs don't default to BoP is because of two things:
1: You can't set world generation to be default BoP for new worlds. This has to be selected during the creation of a new world. This is a vanilla thing, and can not be bypassed.
2: BoP "may" cause some issues on older, less beefy computers, so it's added in the mods list, but up to you to choose it.
Hope that helps!