'mono-valent' vaccines, which were designed for the original or "Wild Type" version...back when they were introduced, they were 95% effective in blocking infection...BUT...as viruses are want to do, they mutated...especially in their 'Spike Protein' and making it easier for them to latch onto human cells (but not evade 'B' and 'T' killer cells)...lots more cases, but still very good protection from severe illness and death...as a consequence, the effectiveness in preventing infection has waned for those 'mono-variant' vaccines steadily. (check out the linked article for a deeper dive into variants).
Because of all those variants it's been tough to zero in on a new target for vaccines, but the recent 'Bi-Valent' version seems to be having significant success, so while no vaccine can be perfect against a fast changing opponent, we do the best we can, and this is it. Your choice is to either take the safe and costless bi-valent booster, or take your chances w/o it...most sensible people choose the former.
Best not to over think this...trust the people who's only job is to keep people as safe and healthy as possible...btw, they follow their own advice...I can personally vouch for a couple of them.
Link: https://www.nytimes.com/interactive/2021/health/coronavirus-variant-tracker.html