There are studies that show, that you do not have much protection against an infection half a year after your second dose. This is true for all available vaccin

There are studies that show, that you do not have much protection against an infection half a year after your second dose. This is true for all available vaccines in the western world. You probably still have protection against a severe outcome, but not against getting infected and thus spreading the virus. A third dose, even of the same stuff, will again increase your amount of antibodies and therefore lessen the chance of an infection in the first place. Using a different vaccine as third dose seems to offer even more protection.