If you purchase online, you will have a choice between getting a paper voucher mailed to you (extra charge for this) or a "Will-Call" number. In either case, you must go to a Guest Relations booth / window to pick up the actual pass. I do not think having purchased online gives you any advantage as far as the lines go. Your year of admission will start on the day you visit Guest Relations to pick up your actual pass. (For many people, that is the same day they first use it to enter a park, but the year starts at pick up even if you do not go to a park that day).
Renewals always begin on the day the original pass expires. Doesn't matter how or when you renew. Renewals must be done within 30 days (before or after) the expiration of the pass begin renewed.
I think there are three advantages to buy in advance.
** One, if you have several to buy, you can spread out the cost over several months. No need to buy all passes on the same day.
** Two, to lock in the price if you think ticket prices will increase before you arrive for your vacation.
** Three, to take advantage of FP+ advance selections. It appears you must have tickets linked to your
MDE account to choose FP+ options.