I've never encountered these before on my other cars. Are these seals something that need to be replaced when doing the plugs? Or are they reusable?

 

I have done plugs on a few imprezas and never encountered them either so I wouldn't worry.

The plugs don't have a seal other then the crush type washer fitted to them already. The only seal is a plug hole seal between the rocker cover and head but you won't disturb this while doing plugs (unless things have gone very very wrong).

 

Ah great news!