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.
Yes, I know that...
but PrimeCoin network does the same thing - so why we need to waste computing power to PrimeGrid?
PrimeGrid exists since 2005. Primecoin exists since 2013. and it's pretty much abandoned today while PrimeGrid is thriving. The very fact that you are a Primecoin fanboi is probably enough proof that it's simply a bad choice, with very slim prospects.
Because NIH syndrom. Its really stupid, better than bitcoin but still stupid.