Yugoslav Attack

It's just a usual move in openings with long castling. The purpose is very simple: to defend a2 and to avoid occasional checks along of c1-h6 diagonal. There are also rooks often operating at c-file. When U are sure that there are no such threats (it's quite rare), Kb1 is not necessary.

Kb1 is a very useful move. There are different lines where White plays Kb1, sometimes at move 12, before White plays h4 and g4, other times after you play h4. Kb1 has different ideas depending on when White decides to play the move, but generally these are the ideas that come along with Kb1 It helps White avoid several tactics that could potentially happen if the King remained on c1. There are usually some Ne5-d3+ ideas where Black can get a discovered attack on an unprotected piece along the 5th rank. Kb1 also gets the white king away from the c1-h6 diagonal which can potentially be dangerous for the King. Furthermore, it is also well known that black doesn't really have a prophylactic move as useful as Kb1 (Re8 is the only real prophylactic move Black has, and practice has shown that Nc4 is better than Re8 anyways). In addition, if and when Black sacrifices by playing Rxc3 to disrupt your queenside pawn structure, your King on b1 will be better placed to deal with that imbalance.