To store message content. To hold message content if you send a message to an offline contact. To handle getting things across networks. To work around NAT etc. To moderate "public square" type features (ie trending posts). Many reasons.
What if one relay is on clearnet and the other one is on Tor?
No problem, relays can communicate cross-network. They relay things between each other so traffic will find a way through as long as one node speaks to both networks.
What if relays I use are not rechable by my contact, that lives in censored country like China and can only connect to relays in there?
As long as there is a relay path between you and your contact, there is no issue. Relays can be run through Tor and other anonymity networks which are very difficult to distinguish from other forms of encrypted traffic.