Why would anyone steal a bike computer off of a bike? Lights can be re-used easily, but a bike computer... they would need to setup their own rigging for the wires and the magnets, that alone would cost them money and time, and who's going to buy from them? That person well inevitably buy cheap, because of the illegitimacy, so maybe $10 or $20.

It plain doesn't seem worth it, so I must be missing something, or these things are being stolen for the joy of stealing.

Is there any value in stolen bike computers? If so how?
I guess there are various different motivations as already pointed out in the comments:


Vandalism. Stealing the bike computer is easy and fast, doesn't require equipment. Lamps (even attached with screws w/o quick release) get stolen, seats (presumably beyond the demand for seats), brake cables get unhooked, quick releases on wheels get opened on parked bikes even if the wheel is secured with a lock …
Potential reuse. Parts for some bike computers can be bought separately. E.g. a Garmin is typically expensive, and mounts and sensors can be bought individually for little money. Now that may not be the case for a stolen bike computer but "Gelegenheit macht Diebe" (german: opportunity makes thieves). One can be tempted to take the computer on the spot without drawing much attention and disappear from the scene, to later calmly in another place figure out if there is any reuse. And said reuse might be reuse of the thief or for selling.