The basic purpose of condoms is to prevent the transmission of infections from one person to another. If a person is reusing a condom with the same partner and they have an STD/STI, there’s a risk that those fluids could come out of the condom and put their partner at risk of infection. Even if they wash the condom out, viruses are microscopic, so there’s no way of knowing for sure that it all came off in the wash.