I've seen no advanntage is purchasing before leaving. As to posts that says you skip another wait in line, we've always been asked at check in if we need tickets. No big line there, everyone has to check in and they ask if you need anything else, real easy for me. Even at the TTC, never seen any lines, we just walked up and bought are AP's no time at all.
Save time.............There is always time on a Disney vacation, we never go to any parks that first day unless we have AP's so there is plenty of time to purchase tickets, ticket media all over there place, every resort sells them, the TTC sells them and every park outside the turnstiles sells them also. We do the DTD shop that first day and our grocery run and visit all our favorite places not needing ticket admission and usually end up in FW Trails End.