It’s very simple if YOU fly the plane at all phases except maybe cruise. If you use all the geegaws and gizmos that interact with one another and the control surfaces and the power settings things can get very complicated not to mention confusing. Especially if something is switched off that ought to be on or on that ought to be off or you’ve input the wrong number somewhere and are getting anxious and behind the flight…
Aviate, navigate, communicate ... as they say.