why pipe fittings are used ?

1.  the material of the fittings should be compatible to the pipe material , mature of liquid flowing , pressure and temperature both inside and outside of the pipe .
2.  pipe fittings are used to connect pipes of same size or different sizes.
3.  there are various types of pipe fittings used in plumbing system serving its purpose .
4. pipe fittings regulate or divert the flow of liquid in pipe
5.   generally they are available in brass  ,  copper, stainless steel or carbon steel .  the different types of pipe fittings used are explained further.( example: steel pipe nipple , elbow, flange)