I suspect it's for a variety of reasons beginning with folks creating an ID years ago for an iPod perhaps then forgetting their username or password when they bought a Mac later or believing somehow multiple IDs on any single device might somehow be more secure, etc. without realizing it offers nothing but pain and suffering down the road when they can't remember what device (or which service on any give device) uses which ID or worse yet, shared one of those IDs with a now-ex-spouse.
Edited to add...iTunes and Photos sharing....creating a "generic" Apple ID just for sharing a iTunes or iCloud Library between family members on multiple devices. Less of an issue these days with iTunes Home Sharing as you can port your shared iTunes library through a Apple TV which family can access or share folders with anyone using iCloud Photos.