You are not required to seed immediately, nor even the same asset. Unless the rules have changed, you would usually just be required to maintain a particular minimum seed lifetime ratio that is easily achieved by idle seeding a library.

The point is to ensure network health with a metric that is simple to understand and verify: that you have been a productive. If you aren't seeding, someone else has to pick up the slack and the network didn't benefit from you obtaining the blocks.

The community itself benefits by giving members a guarantee that stuff there is available with abundant bandwidth, instead of relying purely on the unpaid goodwill of others.