When you purchase an airline ticket, you have purchased a seat from point A to B. There is no guarantee where you will sit.
If you must sit next to someone, a child, a spouse, a friend...
If you need extra leg room, Exit seats, bulk head seats, more leg room seats...
If you are a larger person and need extra room, or an extra seat...
Then purchase a seat in advance! If you wait til day of departure, chances are there will be no good seats left.
Keep in mind, Sunday and Monday are the busiest days or the week for air travel. If you are going on a flight more than 3 hours or if you just do not or can not sit in a middle seat, purchase a window or aisle seat it in advance.
Flight attendants are not allowed to move you on the flight. It is a purchased option so they are not allowed to give it away.
So depending on your budget and comfort, do not wait til day of departure to get a seat assignment.
ENJOY THE JOURNEY!