No, so that the ash doesn't drop into your bong making you not have to clean them so often, or making it so they don't get as dirty as quickly. It does filter out the smoke better but they are more for cleaning purposes, hence ash catcher.
You have ash catchers and pre-coolers, but a lot of people just call them all ash catchers. A true ash catcher does not have a downstem or diffy, just water and it's made to catch the ash that would be falling into your tube. Now a days almost all ash catchers have downstems or diffys in them... these are called pre-coolers. These help a lot in keeping your tube extra clean and they add more diffusion to the smoke making for a smoother hit as well. Hope that helps.
An ashcatcher (by common use of the word, not by definition) is an external perc. Any time the smoke percolates through water, it gets closer to the temp of said water and cleaner. Depending on the type/quality you get, the drag and functionality will vary. With a decent catcher, you can suck a bowl through smoking it to powdery ash without worrying about it gunking up your tube. An external perc is way easier to clean than one inside a 2 foot tube of glass.