These O rings seal the inlet / outlet coupler on the Big Bubba housing body.

We recommend that the O ring should be replaced every 2 or 3 years.

Although they don't perish quickly, O rings can become compressed or damaged due to dirt ingress around the seal.

That can lead to problems getting and the filter system being out of action while a new O ring is obtained - much better to replace them ar cartridge changeover times.