Captains

[The Real Slim Shady]

Sometimes it is simply insurance requirements as specified by the Insurer.

[foxmoth]

You may also find that a particular FO is a much better handler than many Captains (though the Captains skills are adequate) in his company but his management skills are not good enough for command. You may think it then unfair that he cannot handle the aircraft to the same limits (in some companies there is the facility to clear FOs to do this), but it is often found messy to have some FOs with one limit and others with a different one.