Lightning nodes require both inbound and outbound liquidity to route payments, and require inbound liquidity to receive payments.

basically these marketplaces offer tools for node operators to get their desired amounts of inbound/outbound liquidity from the nodes they want to connect with.

bitcoin

Best LN Liquidity Marketplaces

Pardon the question, but what is the point of getting liquidity? Outside of taking loans, is it for developers to experiment? Trying to understand why it's a thing - or maybe I'm just not understanding liquidity in this context.