Some companies want to find how someone would RE their devices, so they pay someone to find a way, then tell them.

Sometimes just a random person will want something REd so much that they'd pay for it to be done (happened more than once)

Sometimes no cause is given to me, which is fine because that is none of my business, really.

Sometimes I just do it for fun, but then I do not get paid, eg: http://dmitry.gr/?r=05.Projects&proj=30.%20Reverse%20Enginee...