In a wafer connection the valve is installed between the pipe flanges and tightened to its place with the bolts for the pipe flanges.
In some wafer types there are some centering holes on the body to ensure valve's correct position between the flanges.

Wafer type body is the lightest body version for mounting between piping flanges. Therefore a wafer valve use to be cheaper than other valves. Also it is fast mounting.

But wafer connection is not an option when one side is removed and is wanted to keep the sealing with the other side.
Neither it is used with hazardous fluids such as hydrocarbons and chemicals. In case of fire bolts could dilated and displace the flanges resulting in leakage.

Valves with waffer connection are quite usual in butterfly and gate valves.