I had been puzzling over the purpose of uploading my GPS tracks, thinking that there was only any point if it happened that someone else found the track and then used it for mapping.
Today, I noticed the button on the JOSM download screen that says “Raw GPS data” and I did a download with it pressed … and I discovered the purpose! It automatically downloads all the relevant GPS data from other people’s tracks so that I can map using both that and my track data.
I don’t recall seeing that written down anywhere - it would certainly have helped me understand as I started.
Assuming my understanding is correct, it is therefore very important to upload all the data you’ve got, so that everyone can use it.