Do drunk driving laws serve a purpose? Every day people driving sober cause accidents that kill people, and every day there are drunks on the road who have no accidents. We don't say "see, it doesn't matter". It's all odds, we reduce the odds. If we can reduce 5% here and 10% there, it adds up.
A vaccinated person is less likely to get the disease. Once one has the disease, one has to get to a certain amount of it before they start shedding it (hence why it is 3-4 days after infection one become contagious and not 10 seconds after). The vaccinated body can start fighting the disease almost immediately, so the ability of the vaccine to get a foothold to get enough to shed is reduced by the body's sudden counterattack.
But then, the danger comes in the initial viral load. The more virus one takes in on that initial contact, the more likely one is to become more seriously ill. Again, if the body is able to start fighting the virus immediately the less likely it is one will be able to shed huge viral loads.
It's the Normandy analogy. The Germans release their armor on day 1, maybe the allies don't get the foothold and don't take the rest of France. The vaccine allows us to release the panzer reserves. Unvaccinated is the "well, let's just wait and see what happens" mentality. Very seldom is letting the enemy attack unchallenged the correct answer. It takes days for an unvaccinated body to know how to fight the virus. In those days, the virus gets a strong foothold and growing numbers without a challenge. This allows the virus to shed, and to shed more. How is any of that good?