Firstly, this prime number is not so big compared to other big prime numbers, such as e.g. the largest known prime, 2^82589933 - 1, which has about 82 million bits.

About the why.. well when we'll meet the aliens, for sure it'll come down to "who has the largest prime..", and we don't want to disappoint.

https://www.mersenne.org/primes/