My understanding is that because of the type of protien that it encodes for, the immunity imparted by the vaccine decreases over time (because of complex immune system reasons). Never to 0%, but lower. The annual booster not only prepares you better for oncoming strains (in theory, when the vaccine research, development, and approval systems work as expected), but re-ups your immunity to existing strains.

The theory as I understand it is that because viruses like COVID-19 pass through populations in waves, your body is developing a very strong short-term immunity to neutralize any immediate "rebound" waves (imagine a wave bouncing off the side of a pool, yes, viruses move through populations like that). It then maintains a weaker, long-term response. By fooling your immune system into thinking you have COVID-19 right now, the vaccine bumps your body ino "short-term" response mode, so your best possible immune response is at the ready if the real thing shows up.

I am not an epidimeologist, but I read a lot of their work from 2020-2023. I might have details wrong, but if it's been >6mo since you've had a booster, you would probably benefit from getting another one.