Because it's fun. I've always had multiple phones from iPhones to Android phones. I like being able to use the Note 5 for work and the S7 Edge for anything else and my 6S+ for music. Last week, I was using my old Note Edge and last night, I was using my Note 4 as my TV remote.
Friday, I brought along my old iPhone 5S just because the thing was so small. Once in awhile, I'll even charge up my old Nexus 4 and bring that with me one day. It keeps things fun and I'm not stuck with one phone an entire year.