My kids will tell you learning and playing every position including keeper is one of the most valuable things for young players.
Over the course of U8/9-U16/17 some kids change a whole lot, growth spurts can see a avg size kid suddenly be on the bigger side, or the formerly taller player becoming avg size. Skills levels can suddenly shoot up or down year to year also. During these transitions primary positions can and do change so best to learn them younger and not try to specialize early on.
When you advance to a certain level, the competition for certain positions get fierce. Forward seems to be the most popular followed by mids, defenders, and keepers.
When you're versatile coaches notice, being able to play forward or defender for example can mean the difference on making a squad or not .
Speed, Stamia, skills are different at different age levels so you always need to work on something to keep developing. With the older kids sometimes that is more strengthening and conditioning certain muscle groups and resting more compared to when they could play back to back, many practices, etc without that much muscle soreness.
YMMV but no one size fits all, some coaches see players in a different light or positions vs the next or previous coach(s) but being flex able and versatile can go a long way.