Yes, and to lower compensation because Boeing’s benefits are probably better than Spirits, and so if Boeing wants to pay the people that make the fuselage less, then having them be employed by someone else helps them pass non discrimination testing for pre tax benefits offered to Boeing’s higher compensated employees.
Also, lower unemployment insurance premiums if they want to reduce the amount of labor they buy, since Spirit would be the one laying people off.
Customers get to decide which engine to fit and negotiate for them separately from the aircraft.
Sometimes there's only one choice, but other times gives you a chance to have the same (or similar) engine to maintain across your fleet (or avoid that to avoid a total grounding) and whatever other reasons to choose one over the other.
At least that's what the A380 tour in Toulouse said. Kinda makes sense to at least keep the big aircraft builders from throwing too much of their weight around.
The discussion about snowman hole issues and other problems is really alarming.