Everytime a virus infects someone, it has a chance to mutate, which is basically an error in it's self-replication. When enough of these mutations occur, it can lead to a new variant in the virus that has new characteristics than the original virus. As we've seen with the new Delta variant, fully vaccinated people can still get infected, but will likely see much milder symptoms than those not vaccinated. If the virus is allowed to continue to run wild in the population, we will likely see a variant that is mostly immune or fully immune to the vaccine, so then we'll be back to square one. Which is why their is such a push to get everyone vaccinated, even those who aren't at a high risk. So basically, the less chance the virus has to infect someone, the less of a chance the virus has to mutate, therefore snuffing out future variants that could be even more vaccine resistant/deadly.