The assumption behind this trade is that Rivers gets hired in Philly - he clearly loves Lou and Trezz (edit: He was indeed hired)

Why Philly does it: Simmons and Embiid are a bad fit and (allegedly) don't like each other; George is a better fit than Simmons around Embiid; They get rid of Horford's contract; Lou and Trez give them depth; Doc can show the world that Lou and Trez can be part of a championship team and have his revenge over the clippers.

Why LAC does it: Simmons is very good (albeit flawed), young and under contract for 5 years; Horford + Zubac help with dealing with AD, Gobert and Jokic (assuming Horford can still be good in a limited role); they get rid of all the players rumored to be problematic in the locker room.