There are several reasons for using a practice pad rather than a real drum when practising. The main ones are:
They are cheaper to buy than drums. Not only is this good for single drummers, but it benefits music schools who have lessons including a teacher and one or more pupils. The teacher and the class can all play together on practice pads, which is cheaper for the music school to buy than a whole bunch of snare drums!
The sound produced is not quite as phenomenally loud as a drum, meaning the family of the percussionist won't go too crazy. Again, this is particularly relevant in lessons where two or more people are playing.
Drum strokes can often sound clearer on a practice pad, so it is easer to hear your rhythm and how well you're playing than it can be with a real snare drum.
Practice pads are generally lighter, smaller and easier to transport than drums.
Types of Pad
Most practice pads are made out of gum rubber or neoprene. They come in different sizes and shapes, ranging from approximately six - fifteen inches in diameter. The most common shape for a pad is round, but it is also possible to find hexagonal and square ones.
They can be bought with or without a stand but most come with the required holes to fit stand attachments, which is useful if you decide to buy one later.
Some practice pads are designed for use as a substitute for a drum. Others, known as drum silencers, are thinner and flatter and designed to be placed on top of a drum to muffle the sound.
Practice pads may come with headphones, which has the advantage of the player being able to hear what they're playing as though it was on a real snare drum while everybody else in the room only hears light taps. It is even possible to buy whole electronic drum kits with practice pads replacing all the drums and cymbals. These are much quieter than real drum kits, but it is possible to use headphones as mentioned above, so that the player can hear a real drum kit sound.
Very small, portable practice pads are available so you can practise wherever you are. These are sometimes made out of a type of bouncy putty.