Single Vision Lenses
Single Vision lenses have only one power. For short sighted people these are negative lenses (thinner at the centre) and for long sighted people positive lenses (thicker at the centre).
Short sighted people need negative lenses
long sighted people need positive lenses
For higher prescription glasses for short sighted people the edges of the lenses start to become quite thick and unsightly and for long sighted people the lenses become thick in the centre. To improve cosmesis and to reduce the weight of the lens a higher index lens (which will give a thinner and lighter lens) can be used. The dispensing optometrist can show you how much thinner your lens will be. For high power lenses choosing an aspheric lens will also help reduce the centre thickness for positive lenses and improve off axis (when not looking through the centre of the lens)
For presbyopic people single vision glasses will only provide clear vision at one distance - therefore it is necessary to have one pair for distance, one pair for near and possibly one pair for intermediate distances (for the TV or computer), however some lucky short sighted people can take their glasses off to read.
The advantage of having separate pairs is clear comfortable vision at each distance but the obvious disadvantage is that the glasses have to be changed for different tasks.