More than 95% of the bands I've been in have had band leaders. The cooperative/democratic model is all well and good when you're 17 and spend hours on end sitting around getting high and fooling with instruments, but when you get out on the real world bandstand that model generally falls apart.
What does the band leader do?
Book jobs
Handle the money
manage band equipment
Set the repertoire
Set the set lists
Call tunes and adjust on the fly as needed
Run rehearsals
Discipline musicians who foul up
Control personnel - hiring, firing
Arrange subs when someone can't make the gig
manage the library (if this exists)
Handle business matters with clubs, record companies (if it gets that far), rehearsal studios, etc.
Front the band on stage (not always)
Obviously in large organizations, those that are at the top of the field, or those that have been around for decades, some of these tasks will be delegated, for example many bands have a musical director that handles the purely musical aspects while the leader is more focused on the business.