So have you ever encountered a faster than light drive in fiction that CAN’T be used in or near a gravity well, at least without serious consequences? A good example of this is Halo 2. A Covenant ship jumps inside Earth’s gravity well/ atmosphere, causes a huge explosion. Other examples of this include the StarDrive universe, the Tom Swift III universe (I think, haven’t read those books since I was a little kid), and a number of others. There is a good reason for this: awesome space battles.
This restriction allows for legitimate, awesome space battles overlooking awesome planetary backdrops. The protagonists need to escape, but can’t jump into FTL in the gravity well, so have to have a running gunfight until they’re far enough away. A civilian ship is falling into a black hole and can’t jump away, so needs rescue. Etc, etc, you get the picture. It’s partially about dramatic timing, and partially about restrictions. Restrictions are what make characters, events, etc interesting. I won’t go into too much depth, since others have explained this much better than I could, but essentially, you make things interesting by laying out their restrictions to be worked around first. If you could jump straight to FTL, the running gunfight would never happen, the civilian ship wouldn’t have to be rescued. On the flip side, traveling slowly through the space between systems is BORING, who wants to do that? (Excepting slowship/sleeper ship/generation ship stories).
You see plenty of non-gravity well restrictions as well. In Star Trek, transporters can’t go through force fields, and battles can happen at warp, obviating the need for a restriction on warp entry. In Battlestar Galactica, jumps are instantaneous, but need recharge/calculation time in between. In the webcomic Schlock Mercenary the teraport FTL can be inhibited by Teraport Denial Fields at the destination. Iain M. Banks’ The Algebraist allows faster than light travel only through fixed wormholes outside of gravity wells. All of these, and many, many other restrictions, exist to set the stage for tense, narratively interesting situations. Sometimes, though, you’ll see it played a little differently, like in Star Wars. They don’t have a particular restriction on FTL, so they set up narrative restrictions instead, notably the Millennium Falcon’s persistent engine trouble.
Why am I thinking about this? Well, because I found myself having a legitimate need to reverse the rule, and have an FTL drive that only takes you to gravity wells, and can’t take you to a destination outside a solar system. Why? Well, I don’t want to talk about it yet, since it needs a ton of work before it is ready, but give it a thought. Brings up some fun narrative restrictions, doesn’t it?