I delete things when I'm done using them
1) Many of my things I either desire to use for many years to come (a video download I paid for), or am required to keep to cover my ass (taxes, logs, most data at work due to policies, etc)
2a) The cost of more storage space is almost always less than the cost of the time to clean up files that could be deleted. In the context of work this does depend heavily on exactly who made the data and their rate of pay / work load - but I've noted the higher up execs and managers tend to be the worst hoarders as well as of course the highest rates of pay. Most of the lower techs on the shop floor don't even have access above read-only to the network storage here, though that is far from universal everywhere.
2b) Yes there are other people whos time is not as expensive, but no one other than the datas owner/creator can know 100% what needs to stay vs what can go (and sometimes even the owner/creator chooses wrong.)
3) After deleting/archiving data, the chances of you needing it in the future are typically higher to much higher than the chances you are really done with it.
4) For the small number of times you really are done with it (like, totally and fur sure), the amount of data that gets deleted is generally such a small percentage of the whole that, while still a good thing to do, doesn't really help much with the problem at hand - freeing up a lot of space for future needs.
I never run out of disk space.
You either have too much free storage space, not enough data, or possibly both :P