Pipe Dope serves many significant purposes when used in piping applications. The main purpose of it is to fill small gaps which may occur when connecting pipe fittings. Using Pipe Dope when joining various pipe fittings also prevents pipe fittings from seizing up, allowing you to detach pipe fittings if need be.
As with anything else, threading on pipes or pipe fittings get nicked, warped, or even chipped over time. Even during production threads can be slightly damaged on fittings or piping. The use of this product will fill in imperfections within the pipe or pipe fitting threading allowing for a perfect seal to be created. For best results, pipe dope is typically applied to the male pipe threading using a brush. As a female threading is connected, the dope with makes its way into the threaded connection to help create a flawless seal.
Connections of pipe fittings often sit for 
extraordinary amounts of time before needing to be disassembled. Many times when a threaded connection has sat, experiencing various amounts of pressure, they will seize up. Applying pipe dope prior to connecting pipe fitting threads will work as a lubricant, allowing for easier removal when necessary.
Ensuring you use the proper type of pipe dope when connecting pipe fittings is very important. There are certain types of pipe dope designed for PVC piping versus stainless steel pipe fittings or galvanized fittings. Using the improper pipe dope can cause a seal to break down quicker and create leaks.