PrimeGrid purpose is to compute huge prime numbers, these numbers are useful because they are the basis of encryption, just by connecting to this site you are using them, with SSL using this https://en.wikipedia.org/wiki/RSA_(cryptosystem)#Operation to secure the connection. It just happens that to this day we already have enough prime numbers, and it would be much more efficient to find them 10 years in the future when we do need them.

Also Quantum computing can potentially break this algorithm in seconds. Its literally the only application they have to this day. If we managed to make them work.