Well it provides them with sMerit, with more sMerit to vote for other posts if I may say this. While I, myself and I DON'T HAVE THE ABILITY to give merit to others. So if you are an "old schooler" here you have more power in your hands. Same applies with getting this "merit source" status you have to be high rank to be able to give merit to others and this is not related with our knowledge or engagement within the community.
It's a one-time bonus for existing accounts, and only for those that were active within the last year. Once those sMerits are gone - they're gone. New ones have to be earned the same way for everyone, rank has nothing to do with that.