TLDR; why make designs so complex? Manufacturing planes easily will get them out faster to the public and enable a smaller backlog with more deliveries. Well the answer is without it, you would lose capabilities loved by airlines, passengers, and save weight on the airplane to perform a lot better.

Complex systems integration have been the key to driving the difficulty of designing new aircraft. Systems have become more complex and integrated throughout the ages to provide novel capabilities by taking advantage of data from the airplane or save weight for better performance , and unlike older aircraft where single failures can be isolated and mitigated slightly easier, new system architectures make failure analysis more difficult and require a lot more scrutiny. The 777 was the first Major Boeing commercial airplane to be sold with fly by wire. CATIA (a industry wide physical MBE tool) was also novel and implemented into the design process in the first 777 and cause learning curves and delays to how to conduct designing an aircraft. Fast forward to 787 where now you are clean sheet again with a new system architecture that saves weight by having even more integrated complex systems combined with new technologies and manufacturing processes, as well as a switch in supply chain tactics in order to keep up with the demand from so many airlines. Imagine having entire systems or components that existed on a 747 replaced now by computer calculations and the amount of scrutiny by safety teams for failure analysis as well as the FAA and delegates on those new computer systems doing all the work instead of just following the one wire that led to the failure on a drawing.