....around the bottom of my valve spring? Some kind of retainer?

That's a form of spring dampner, found on some of the GM engines like the mark V 454's and if I correctly recall some of the V-6's and maybe 4 cyls.

If I wanted to use these springs on olders heads, do I need them?

They likely won't fit if the other heads have a tight fitting spring pocket.

The whole thing is on a seat that fits inside the pocket.