Does the ATPL theory help with day to day flying? Not really. If at all.
I guess it's useful to know a little about Principles of flight, performance and navigation, but really it's all geared towards a 737 type aircraft. Pof, perf, nav and met are all done at PPL level and that's what the hour building is done on - so you don't strictly need the extra stuff. It's all good background info though. Honestly most of the stuff in the ATPL exams is detail you'll rarely need in day to day commercial flying. Just hoops to jump through.
Wishing now I'd booked an aircraft for today...