i have lots of reasons!
one is the inherent tinker factor: there's always something else that you might want to try to improve the printer. (yes, silly but fun...i'll document my own progression from a small chinese kit pinter to a prusa/3030 homebrew in a separate post later.)
for me the ability to build a custom part without too much hassle (most of the time...) is what makes the printer so awesome.
here's one: you want some custom hooks to gather your kitchen curtains out of the way? easy. 15 minutes with openscad, a little bit of PLA plastic filament, some printing time later and two pieces of double-sided tape and you're done.
or this one: you live in a climate that means any dust in connectors will cause them to corrode and rust into oblivion pretty damn quickly? easy. design and print some connector caps. oh, you'd like some that are angled at 85° to fit the sloping sides of your lapdog? that's just a small tweak in the design.
or this one: your daughter wants a transparent base for her wedding cake and roses underneath, but doesn't like any of the commercial supports available; so you're asked whether you could design Something Cool, and you try, and it works out pretty damn well:
or: you want some simple but functional hooks, to arrange your extension cords in your laundry's cupboard? no problem.
or: after a few unsatisfactory iterations of double-sided tape and other hacks you want something not too ugly to help you park the power supply for your kitchen led strips underneath the cabinet; well, there's a general-purpose customisable mounting hook design on thingiverse and it works really well.
or this fun reason: your colleague manages to drop her whole key ring down the lift shaft (13 stories plus a few underground levels for good measure), so you think it'd be fun to rub it in by 'helping' her with a glow-in-the-dark never-sink-em personal key buoy (guaranteed to not fit into the gap between the lift car and and the floor!).
eeeasy! less than 50 lines of openscad.
on a slow evening you think about making parts for a custom pendant/mobile for your daughter? can be done even on a small printer.
or this, very recent, typical case: your old but nice burr grinder loses the built-in deflector hat, so the beans tend to jump around rather than vanish down the Maw of Future Fresh Coffee? no problem! but it might take you two tries until you get the size right, as the old deflector is long gone. (the translucent orange one was my first print using PETG plastic today, after years of using PLA and ABS.)
one of the early reasons: you inherit a nice bicycle mount with a led light, and the led light is dead; but as you dislike waste you want to use the mount for other stuff? sure, doable and not even hard; eventually i made mounts for my new light, a spot messenger, and others for a small 808 action cam and for my mobile phone (not shown here).
or a toy-related one: your very custom wheely king crawler conversion needs a bumper to become kid-safe again (the previous ones made from shapelock blobs didn't last long enough), so you design and print one:
or this reason: you end up with some surplus LM2596 stepdown modules and decide to make a few whatever-to-5V converters (for car and camping and the like) but with decent heatsink please! which, of course, is only good when it's housed in a not totally shabby case:
occasionally i'm also lazy and don't design my things from scratch (or with inspiration from others' work); every now and then i just take a design off thingiverse or elsewhere and just reify it...like this digital sun clock or the headphone cable organiser, or the case for this raspberry pi.