It’s to help increase the privacy of lightning usage. Using a coinjoin spend to open a lightning channel aims to obscure the ownership of the utxos going into channels.
Right now utxos which open channels can be tracked by chain analysis and then btc moving through the lightning network can be tracked with high degrees of probability
But how would they track the movement of funds all the way through the Lightning Network? I thought each node involved in a transaction only knows the node immediately before and after it in the path?
most traffic goes through large public nodes and only involves a couple of hops. if you’re tracking the large, public node in the middle and you see 1M sats go in and 1M sats go out within milliseconds you can have high degree of confidence theyre connected
similar to how people are tracked through tor, ie timing attacks.