The fact that ugg boots are designed to be worn over trousers also makes them practical for a lot of reasons - for example meaning that they won't let your trousers drag along the floor and get wet. In that they make your feet look larger, they are very much like flares except will have the opposite effect of protecting the bottoms of trousers.
Of course any cheap ugg boots will have all of these advantages, so what is it that makes sheepskin ugg boots in particular more practical? Well on top of all the aforementioned bonuses, with sheepskin you also get a material that's highly resilient and won't get torn and that's highly easy and practical to wipe down. At the same time the standard beige colour is one that won't show dirt that easily meaning you can subject them to a lot of rough and tumble and still come out of it looking good.
And sheepskin ugg boots do look good, and make great shoes in that respect too. Any pair of boots will have the desirable effect of sliming the leg and hiding any bulges etc - calves for example will be hidden if they are normally a bit too bulky thereby creating a highly desirable silhouette. At the same time though the ugg boots will improve this look by making the upper leg look thinner by comparison.
The increase in popularity of sheepskin boots is due to more fashionable designs. These days in particular womens ugg boots are become more of a must have fashion item.