Prater's objective is to ensure that the network remains stable under a higher load than we've seen so far on mainnet -- the genesis count for Prater was 210k (almost double the size of the Beacon Chain Mainnet).
To elaborate a little, we want to make sure that the network is able to function properly with considerably more validators: increasing the number of validators increases the state size, increases the amount of work done to process that state, and increases the number of messages being gossipped on the network; blocks also become fuller, which explores a new kind of constraint as clients need to optimise better for attestation inclusion.
