Silver is bacteriostatic, which means it can kill or incapacitate bacteria. This reduces the risk of bacteria growing inside of the housing.
This is useful for caravan water filters, as most people only travel intermittently. This means they might leave water sitting in their water filters for several weeks between trips — which can give bacteria time to grow.
Even if you are using silver impregnated filters, it is best to never let your filters go beyond 2 years of service life.