Why shouldn't you let bios configure it for you?
Because you have no idea what the BIOS is changing and when and where and how. It could be altering your vcore or memory timings while you are Windows, or playing a game or something and will most likely result in instability. Plus it most likely won't like going very high and you won't get nearly as much out of your overclock.
Imagine having something "automatically" driving your car for you. Well, that's my thoughts on a nonuser defined BIOS configuration. In all honesty, if you don't know what you are doing to a point where you are letting the BIOS do it for you, ya really shoudn't be doing it in the first place.