Since this is a "self-extracting" crank arm, that cover plate is needed for the fixing bolt to press against as it is backed out. If that cover plate unthreads with the fixing bolt, you can't remove the crank arm. The pin holes are there to allow use of a pin spanner to keep it in place if it starts to loosen.

BTW, greasing the inside of the cover plate and the fixing bolt head will help assure the cover plate doesn't back out with the fixing bolt in the future.