Based on your wording, it sounds like you're conflating the two things together... running an exit node and using tor are orthogonal to one another both in value provided to the user as well as effort involved.
Plausible reasons for:
(a) you greatly value privacy and the privacy of others such that you are willing to altruistically provide an exit node as a service; your country is a police state and you are sympathetic to those affected while also willing to accept the risk
(b) you greatly value your privacy and do not trust your ISP; you cannot access content sanctioned in your country; you are an internet engineer and need to test services which depend on privacy as a core feature
Also, I assume it wasn't intentional, but consider against arguing from the position of "I can't think of anything". You are betraying yourself by implying that you know all there is to know...which no one does.
Maybe if I'd said "I can't imagine X therefore Y must be true," but what I said was "I can't imagine X so I think Y" which doesn't imply I know everything at all. Quite the opposite imho. It implies I could change my opinion on Y if you showed me an example of X.
In any case, apparently that plus my use of the word "not" were ill-advised, since almost everyone here misread my comment to mean the opposite of what I actually meant. One person did read it as intended, and got another person to see it that way.
As someone else helpfully clarified for me, my point is that anyone with the technical skill to run a Tor exit node is also going to use Tor to hide any illegal activities they do online.
Plausible reasons for:
(a) you greatly value privacy and the privacy of others such that you are willing to altruistically provide an exit node as a service; your country is a police state and you are sympathetic to those affected while also willing to accept the risk
(b) you greatly value your privacy and do not trust your ISP; you cannot access content sanctioned in your country; you are an internet engineer and need to test services which depend on privacy as a core feature