Purpose to act as validators: yes, validators of consensus rules, so nodes don't waste each other's bandwidth and computation processing transactions which are not consensus valid and so cannot be mined. It has other purposes, specifically, that you know how much fee to assess on your own transactions so they have a high likelihood of getting confirmed, and that, by spreading the most likely set of txs to get mined widely, the process of validating blocks can be done quickly by everybody, and especially miners. Don't forget: it is entirely possible and logical (if you're in a constrained environment) to run a node with no mempool at all (blocks-only). It is *not* logical to try to filter out transactions which are consensus valid, and pay market fees, but you don't like for some reason, because all of the previous points break, *and* it is not at all effective, because bitcoin's network is a gossip or broadcast network. Its entire point is to prevent the censorship effect you're trying to achieve by filtering out transactions that are valid, but you don't like. There is no getting around it: if you think a certain type of transaction should not be allowed, you have to campaign for a soft fork to prevent them being valid. Using the mempool for this is a confusion, and a very unhelpful one imo.