It’s normal to need boosters for vaccines. (Think about all the boosters you got as a kid, for chicken pox, tetanus, whooping cough, and more.)
The reason you need them is because the vaccine doesn’t stay in your body. After triggering your immune system to start naturally producing the antibodies necessary to fight the disease, the vaccine dissipates within a few days.
As time passes — and if you’re lucky enough not to be exposed to the virus — your immune system starts to “stand down,” creating fewer natural antibodies. However, with the virus still out there, the danger hasn’t passed. Boosters are a safe way to tell your immune system to keep producing antibodies.
Boosters can also be improved as new virus variants arise — so they give you added protection against the virus as it mutates.
https://health.clevelandclinic.org/covid-booster-shot/ (Cleveland Clinic)
https://www.yalemedicine.org/news/covid-19-booster (Yale Medicine)