In the majority of bathrooms around the country, the space under the sink is home to nothing more than pipes, bends, and stop valves. These pipes are almost exclusively surrounded by a large and obtrusive facade that, though it may hide the ugly plumbing, does nothing to increase the available space and aesthetics of the room.
Vanity units make use of the otherwise wasted space to be found under the bathroom sink. These units also have another important advantage over the option of a fixed facade to hide the pipework - in an emergency, you'll have to tear out the facade to get to the pipes, while the majority of vanity units come complete with a removable panel behind the cupboard, thereby allowing access to the plumbing without compromising the integrity of the unit itself. This is an important point to note, as insurance claims for damage caused by burst pipes are in the magnitude of hundreds of millions of pounds per year. A huge number of these claims could be prevented by quick access to the pipes behind the cabinet, so investing in a right unit may well save you a lot of time and money in the end.