Just wanna do a quick survey here. What is it that makes you develop Moodle plugins?
Are you asking: "What is it that makes you eat foods?"
Survey for what?
My reasons for wanting to develop plugins (as well as William's superb answer) is to get Moodle to do what we need.
Others may say
I'd add to Derek's list:
a. because its my job (either for clients or for my institution - at the request of users/employer)
b. to learn (how I got started with themes - as a type of plugin - to learn about html/css/etc. I think I published about 1 in 10 of the themes I worked on when I got started!)
+1 to William's answer
I would add:
* to make moodle more useful
* to prototype new features / technology that might get into core
Writing plugins is cleaner for maintenance than hacking core. And there's also: belief in the power of collaboration. My institution benefits tremendously from plugins that others have developed. We shouldn't all be repeating each other's work in isolation. If I can fix a bug upstream or publish a plugin, then we are giving back.
adding to everyone's else's comments...
collaboration, working towards same or like goals in mind. learning, teaching,