It has value as a cryptocurrency alone, it doesn't require proof of work for creating blocks thus is far more energy efficient than Bitcoin & it has faster block/confirmation times.

Someone may buy GRC to speculate with, another may buy GRC to obtain a larger vote weight within the whitelist polls in order to get their project whitelisted in the future.

Some buy GRC to then rain onto Gridcoin/BOINC users for their completed work.