We use them based on many different factors. We use Hikvision equipment.
For starters, we can get a strobe light, microphone, AND speaker in a turret in the PCI series. None of those available in a dome.
Secondly, turrets tend to handle rain better. We have an installation with a mix of domes and turrets, both in fixed AND PTZ variants. The flat surfaces of the turret style cameras seem to pull water off more easily, whereas we are finding the dome-style PTZs will retain water drops more readily and obscure the image. We plan to experiment with spraying them with some RainX to see if it helps alleviate the issue.
Another benefit, in addition to what everyone has said about aiming them being easier, you cannot under any circumstance aim a dome camera's lens higher than 90 degrees unless you mount it on the wall, and generally they will top out at less than that like 75-80 degrees. A turret generally allows you to exceed 90 degrees if necessary while still being mounted in a vertical orientation. Overall its freedom of movement is generally better.
And speaking of which, turrets are MUCH more suited to wall mounting versus domes due to water intrusion as well as streaks and water drops that tend to collect on domes mounted this way. We prefer to use wall mount brackets for vertical orientation even with turrets, as they generally give a more professional appearance and allow for MUCH better upgradability. However there are times when directly wall-mounting the camera makes more sense.
One instance where we DON'T like using turrets is where the prevailing winds will expose the lens to blowing snow and frozen rain. Due to the aerodynamics of a dome it allows these sorts of weather conditions to bypass it and will not collect snow or ice, however the flat face of a turret (or bullet, for that matter) will most definitely do so. In some cases we've had turret-style PTZs get so badly iced up that they will no longer move and require a service call with a heat gun to rectify. Where cameras may face these conditions, which here is generally facing northwest, a dome style is much preferred.