These tools smooth drywall compound on walls and ceilings to reduce the number of lap marks and leave a professional finish. You can use skimming blades on other types of compound too, like plaster, for example.
They also come in handy for butt joints.
When Should I Use a Skimming Blade?
You should use a skimming blade to add a skim coat to a wall. A skim coat consists of diluted joint compound that’s smoothed on the wall or ceiling to acquire a level-5 finish. (Level-5 is the premium finish of drywall.)
Here are some examples of when you might need to skim coat a wall:
You’ve recently installed drywall and want a level-5 finish.