If you stay away from people when you're sick that definitely helps. The only drawback that is you may be contagious before showing symptoms.

Getting boosters help reduce spread and reduce risks of complications from viruses (like getting an annual flu shot or Tdap boosters). Not everyone can get the vaccine (i.e. they're allergic) or individuals may have weakened immune systems and their body can't produce enough antibodies. If the community continues to test and get boosters, it helps everyone stay healthier and potentially save lives.

Just because it's endemic doesn't mean there's no more action needed. Hospitals may not be packed like they were (although they still get busy with COVID during peaks stressing the system), it doesn't mean that you can't help protect others.

One of the positive things that came from this pandemic is at home tests (not just for COVID). By testing when sick (whether it's the flu, COVID, rsv, etc... I've had COVID where I really thought it was allergies. Was about to go out to meet with friends and figured I'd test just in case. Sure enough I was positive with what felt like mild hay fever (it was summer time). I'm just glad I tested and didn't get anyone else sick.