So, good question. The wait for the monorail can be pretty high (I have seen 20-30 minutes on weekends). Which means, if you are all young and walk pretty fast, it tends to be faster to go through the main gate and walk all the way to Tomorrowland.
If the app says the wait is 5 minutes, than it is faster than walking to the gates.
Maxpass reservations can be done once you go throught the turnstiles. That validates your tickets.