Side Quests are a key part of any RPG, or any quest based game really, that you can find on the computer or favorite videogame console. They are used to explain back story, define setting, and otherwise breathe life into a game. The thing is, I'm not sure the term works for table top RPGs. Today I want to discuss that.
A Break From The Main Quest
The most defining part of a Side Quest is the Side part of it. A side quest by definition is not the main quest, nor does it have any impact on the main quest. It may give you more information on what is going on, sure, but it doesn't impact or progress that story.
This is also where you get the big problem with side quests in table top RPGs. What is the main quest? In some games that's known. If you're running Hoard of the Dragon Queen then you have a main quest. If you have an over-arcing metaplot, you have a main quest. But if you're running a campaign "the PCs go around and do what they want" type game, you don't have a main quest. You don't have side quests either. You just have "things the PCs can do."
Pausing The Action
The second problem side quests run into is that in videogames and computer RPGs unless you're doing main quest missions it is almost like time is frozen in the world. The example that always sticks out for me in this is the old Playstation 1 RPG Suikoden where at one point a girl is kidnapped by a vampire who will turn her into a vampire like him at midnight. Upon hitting that point, my friend promptly went to the inn and took a room for the night to refresh his Hitpoints and Mana, and then went off to several other areas of the world to gather equipment and get some levels - staying in several other inns along the way - before returning what had to be 14 days later to arrive just in time to save the girl from being turned into a vampire.
Now, some computer RPGs don't do this. The Quest For Glory series is happy to end the game if you don't solve a problem by Night 6 if that is when you have to solve it. But for the most part it is true, and it is true even in modern RPGs unless a specific mission has a timer on it (looking at you save the crew mission from Mass Effect 2.)
Compare this to a table top RPG though. The GM is the ultimate gaming CPU you can have at the table, and its job is to dish out consequences to actions. If you're given a night to save a princess and you decide to go on a 2 week vacation before trying...well, guess who is finding a vampiric princess when they get to the top of the tower? Have a dire mission of a pending invasion, but you just have to go off and scan half the galaxy for plutonium and Element Zero? Welp, guess what's happening while you're off in the middle of nowhere? Yeah, that invasion.
This also limits side quests in table top RPGs because the world doesn't pause while the new Hero of Destiny goes off to find some farmer's horses, or take up a job at a farm while trying to figure out who is stealing the cows at night.
A Source for Extra XP
The last point of sidequests in most computer RPGs is to give the player XP in order to level up for the next encounters. The idea here is that the developer has an expected level the PCs will need to be at to progress the story, and getting that XP will require doing some things that aren't on the main quest. In other words, some of the side quests have to be done - and the player in turn has to do some world exploration - or they will be under leveled.
This can be done in table top RPGs but it generally has bad results. No one will blame the GM if a bunch of PCs get wiped taking on a CR 15 Dragon when the party is only level 10. However, when the story pushes and rushes the PCs into that encounter and the party is underlevelled because they were supposed to ignore the portents of doom and do some side quests and random encounters to get more XP? That's just considered bad game design.
In a computer the world will wait for you to hit level 15 to unlock the next dungeon. In table top, like in the real world, time doesn't wait. Which in turn can kill sidequests.
In The End
In the end I suppose my initial statement was off. It's not that you can't have sidequests in a table top game (provided you have a main quest) but often times you probably shouldn't...or if you do, you need to shape your story to allow for them to fit the feel of the rest of the game.