I love to send players on a quest. To me, one of the best things about AD&D is that you aren't locked into one "game" or module and that's it. Module is completed, everything is done.
Oh No No No No.
Quests can help define the game itself. You can loop together several pre-published modules and home-brewed games all under the auspices of a quest.
A quest can be a grand one, taking weeks, months or even years to complete. They can also be short, taking all of one sitting or game to complete.
With a quest, aimless wandering is now replaced with an objective, a goal to keep players moving in a given direction. Not that there's anything wrong with aimless wandering and adventuring either. That can be plenty of fun in it's own right.
Questing is a great way for a DM to recapture certain players interest again during a campaign. it takes what may have been becoming a bit dreary and pointless into something with a tangible outcome. Something with the promise of great challenges and great rewards.
It could be something so simple as finding a message in a bottle where the sender is seeking to be rescued or sent something valuable away from themselves to keep it out of a villains hands and now the PC's must find who sent it and return it to them.
Perhaps a King or even a god has approached the party with a quest and has compelled or promised great rewards for the finding or returning of something to it's rightful place or to the possession of the King or deity.
Perhaps a player character has been enchanted by an object or a magic user into going on a quest for something and until the item is obtained or the enchantment is broken, they are compelled to follow it through.
So many options and possibilities.
Just one of the reasons that AD&D is such a great game.