think of it another way, if you start on maybe a new tune every day when you arent really that experienced yet,you are playing around,and might wanna leave it alone after some time,till you got the skillss of making it sound like you want it to,or even just that you get bored of it or dont know what to do with it at the moment. months( maybe years) after you go through your unfinished things,and you can maybe see a much better potential for some of the stuff + youve gotten better technically, so something new comes out of it. after a while you got so much better that you are able to take all those tracks and one by one complete them.
i saved almost all i made except when i thought it really reeked, and dug a lot of it out a while after and made a great thing outta it,fitting to what your into at the moment,etc.
ive been doing music for a few years now,and the whole first year i wasnt able to finish a track,but the last ½ year ive been able to finish a lot of thngs,gradually, because of this
i can see that if you have a release deadline or so its really necessary to be able to finish off,but if you dont: just play around. cause one day it will be there,in one way or another