If there are many different strains loose in a region that don't confer immunity to each other and immunity only last a year people could be perpetually sick with the virus. A vaccine combines a variety of weak or inactive strains that give your immune system a big picture view of the virus to develop a comprehensive response to COVID-19 instead of COVID-19 Strain 93A.
The flu vaccine works the same way: scientists predict which strains are the most likely to spread and create or find weaker strains that are all combined into a single vaccine.
That's an interesting and important point. Luckily it seems to mutate 10x slower than influenza viruses and so far no different strains have been found. Finding new strains would also thwart some of the current work on vaccines being done.
I was being a bit imprecise when using the phrase "strain." In the context of virology in general, a strain is delineated by a difference in functionality - i.e. some key component of how it infects, replicated, or damages the body is different from other known cases. However, in the context of vaccines, a "strain" may function the same but might not produce some unique protein or other marker that the immune system uses to identify the pathogen, without having an effect on the rest of the virus.
COVID vaccine work is still in its infancy so we don't know yet whether the non-functional mutations we're seeing in COVID's phylogeny will effect how effective vaccines are.
The flu vaccine works the same way: scientists predict which strains are the most likely to spread and create or find weaker strains that are all combined into a single vaccine.