Why both? Lots of RFF discourse on this, of course.
My positive rationale: I enjoy both, and I benefit from changing gear/shooting /development habits, refreshing vision via mixing up the optical tools. I often use a digital/film buddy system, too (this past week of travel, an RX1 and a Klasse S). I have a trusted film development pro in Oregon, and can scan/print here, or get exhibition IQ from Photo Lab in Berkeley.
There are also the negative rationales. First, having image duplication or overlap in emulsion, because I can predict no better than anyone how digital image devices and storage will evolve, crash, be wiped out. Second, I have not yet solved the conundrum or liability of boredom (mine!) that comes with an ascetic/minimalist approach. During my one-cam decades (OM SLRs), I photographed less. Which mirrored how, in my one-guitar decades, I gradually practiced less, composed less, etc. Indeed, I learned less and was less interested in learning than I have been since adopting a variant of Love One Woman and Many Cameras/Guitars. But that’s another positive rationale, isn’t it? And a delightful one.
Also, I have a bunch of freezer film to shoot. ;-)