This puzzles me because I finish something, bounce it and then I'm done. I move on. Last week's work is done and forgotten about like last night's dinner. Often I delete the project because I know I can recreate it quickly again if need be.
 
Why do people hold onto DAW music projects and test them with latest builds when they are several years old?
 
What is going on???
 
To quote the great Gottfried: WHO ARE THESE PEOPLE!!!???

Suppose George Jones or Brian Eno called you and said, "well, I'd like to produce your song, but we need some changes in the mix.  can you send me the stems?"
 
Happens to me all the time. 

^^Good answer :o)
I keep the projects and go back to them in order learn. Sometimes I find my 5 year old approach to something actually sounds better than some new one I've been excited about. Or I learn a new way of using a new VST and want to try if that works for some older problem spot, too. etc etc.
I guess I would not act like that if I were a professional, selling my music, having deadlines and lack of time. But anyway, I would not delete the projects.

I use old projects to test out new software. Is that new de-esser all it claims to be? Let's see if it does better than the sidechained compressor I used 5 years ago. Is my new string library the cat's meow? Let's drop it into this old project and see if it's really better than the Dim Pro patch I used 10 years ago.
 
Sometimes, hearing an old project yields an unexpected revelation. Wow, the reverb sounds good in that one, let's look and see what I used on it. Pantheon? No sh*t? I stopped using that in 2007 because I thought it was crap.
 
Pulling up old projects can also be good for one's self-confidence. Maybe the project sounds pretty good considering you didn't have any FabFilter, iZotope or ValhallaDSP stuff at the time, which gives you hope that you might actually know what you're doing after all.
 
Or it can be depressing, because even with all your subsequently-added Kontakt libraries your new stuff doesn't sound any more convincing than it did a decade ago. Or you realize that today it would be impossible to replicate the high harmony you sang back then, at least without hiring a female vocalist.
 

To be an ever bother to the bakers?
 
I have some stuff from waaaaaaay back just waitng for the Nov release.

I have projects that I'll never finish because I just seem to enjoy tinkering with them, getting pissed with the way they sound and then reinventing them into something new. I have one that's 3 years old that I still fire up from time to time just for fun. I don't even care if I finish it or not, will be a nice surprise if I do, but I'd like to think my DAW could always load it without a hitch. 

I keep most of them because I'm almost never actually done with them. I have some songs I've recut and reimagined in many different ways. Eventually most finally "click" and are finished...but not always. I work the same way with my painting and writing projects. Go figure.

I can't toss them out. 
I have learned to convert all tracks to waves files so I never need to rely on midi or internal instruments. 
It's rewarding to go back and remix them which I have recently done on a couple songs. 
I use a 1 trig HD for all my music files which gets backed up to a 1 1trig backup drive so I will hopefully alwasys have these old guys around.

If my "30 year safe storage CD/PD" hadn't disintegrated taking all my older work, I would have LOVED to have it still around for all sorts of reasons.
 

backwoods
To quote the great Gottfried: WHO ARE THESE PEOPLE!!!???

I'm curious about the opposite, besides you (obviously), you DOESN'T keep their old work? 

I keep old projects because I like to open them up and listen to them occasionally, maybe tinker around with the mixes a bit. It's nice to have a record of stuff, and it takes up very little space. Sometimes good to revisit guitar parts and things and get ideas for new projects. Not sure what would be a good reason to delete it all really.