My understanding is those who have more coins on a PoS network have more stake / power. This can matter if there are things like on-chain governance / voting rights, depending on their stake. Those with more stake would also get more staking rewards (it's like an APR % return based on the total staked), and if they stake their rewards as well, then they'd have even more total stake on the network. There also are concerns with those having a majority of the stake in a network being able to disrupt or attack the network (things like slashing based on poor behavior can prevent bad actors from wanting to do that, as they would lose some or all of what they had staked in that case). There are also different kinds of PoS though, and I'm not an expert on it.