I want to know how exactly people reason out what feats to take at what level and especially what class they level in at certain levels. I.E rogue lvl 1 and rogue at lvl 8 etc. Basically the question comes from the reasoning why at lvl 1 and 8 and not 1 and 2?

That is just a example. I want to know the overall reasoning behind why people take X bard levels at lvls A B C, then X rogue levels at D,E,F and X Barabarian levels at G, H, I etc etc.

So what is the reasoning for taking
Rogue levels at certain levels
Fighter levels
Barbarian levels
Ranger levels...

you get the picture.