Why The FCC only tells you which frequency ranges and modes each license class may use; it does not say where within a band to put CW, digital, SSB, FM or satellite work. Band plans fill that gap as voluntary agreements, mostly published by the ARRL and coordinated with other groups, so that narrow modes and wide modes do not land on top of each other. Following them is a matter of courtesy and good engineering rather than law, though ignoring them is a quick way to cause interference.
Watch out The choice about a club devising a plan is close but too narrow: band plans are developed by national and international amateur organizations for everyone, not by individual clubs. Net frequency lists and operating schedules are just directories of specific activities, not a structure for the whole band.
FCC says what you MAY do; the band plan says where hams AGREE to do it. Voluntary, not regulatory.