When you simply can't get anywhere the normal ways
On some sims, you have to "cheat" to get around the place, either fly (if that's allowed) or use the map to teleport around (if you can see anything on the map). The reason is because the interesting and/or important places on the sim can't be walked to, nor are there teleporters that'd take you there.
Usually, the sim owner doesn't know because they only ever teleport everywhere with their personal TP list or by use of the map. Or they simply fly everywhere. They've never walked anywhere on their own sims, so they don't know that it's impossible. And if they don't need a network of teleporters, they don't notice that everyone else does if walking is already impossible.
Seriously, please build your sims in such a way that at least all important places can be walked to. If there's water in the way, either build a bridge. If that's impossible for whichever reason, install two boat rezzers, one on each side. If you build pathways and bridges, walk them to be sure that they can be walked. If they can't, fix that.
You may also want to install a network of teleporters. They're useful for the impatient, for those with no sense of orientation and, of course, if places on your sim absolutely can't be reached by foot.