3 ideas
Even if every possible action happens in some branch, choices still matter because your character decides how many of your branches contain the good act rather than the bad one. A generous person is one whose paraselves mostly act generously. Building better habits therefore changes the proportion of good outcomes across the multiverse, not just in the timeline you happen to be in.
Once people can talk directly with the self who took the other path, regret stops being private speculation. It becomes a concrete comparison with a specific rival who looks like you and is doing better. That constant benchmark causes fixation, resentment and paralysis, which is why the prism world needs support groups to help people stop measuring themselves against paraselves.
Each prism holds only a limited amount of data for talking with its linked branch, and when that supply runs out the connection is gone for good. That finiteness turns contact with a parallel self into a commodity people can buy, sell, hoard and use in scams. It also forces users to choose what is worth saying before the channel closes.