session is basiclly federated (the same model as email and fediverse), where there are different nodes for session. and unlike mail fediverse federation, (if i am not wrong) they have balanced nodes, so basically traffic is spread rather uniformly. as for motivation for starting a node, there are not many, nodes basically form a block chain of sorts and also have something like tor (called lokinet) which also makes your comms hidden.

i personally never got into session, because it seemed like too hard and complex (i do not like using complex software. even if i do not read to run server nodes for it, i should atleast know how it works to understand my safety model). and unlike signal (session started as a hard fork of signal afaik), they broke pfs (perfect forward secrecy - basically something magically, which makes it so that if some bad actor broke encryption for one of your messages, they can not do that for next message)