Immunoprecipitation compared to western blotting
The two techniques serve different purposes:

IP: purifies a protein

WB: visualizes or quantifies a protein

Most often when I have done IPs (in the hazy past) I have turned around and run the protein out on a WB, so one will frequently combine both techniques. Sometimes a protein is very low abundance so you will not see it in a small volume of crude cell lysate on a WB, and you have to IP it from a large number of cells to get enough protein to visualize on a WB. 

Note that IPs will also co-purify associated proteins, so a good way to know whether two proteins stick together is to IP one of them and probe for the other on a WB. This "co-IP" analysis is a very common use case for both techniques.

Hope that helps.