Any private detective will tell you - never be surprised where your investigation will end-up. The world may seem like a big place but when it comes to investigative work, you can end up anywhere.
If you're a private detective working as a one, or two-man agency, you have to have the support of other investigators. It's inevitable at some time during a PI's career they will need help.
Starting a case in Idaho State can end up in another State, of which has its own set of laws governing how a private investigator can work. So, if you're an Alabama private investigator and you track down your quarry across State to Arizona, you will need to get the assistance of an Arizona private investigator.
You could cut across State to continue the investigation, as long as you are also licensed for carrying out investigative activities in that state, but is that being efficient? And does your client have deep pockets for the additional miles and flight tickets?
Building a network of good trustworthy investigators you can call-upon when your case needs it is beneficial all round. It benefits you through networking with other PI's and possibly getting work handed back to you, and it benefits the client, having reduced expenses.
The other main obstacle that gets removed by PI networking is the problem of not knowing the laws of the land you now find your case is in. By handing part of the case over to a local investigator you can work on other jobs while following progress of the out-of-state case.
Also, the private investigator you've hired to catch the cheating spouse will know the lay of the land alot better than you. The local PI will probably have much more success in progressing your case than you would, vica versa, if the shoe were on the other foot.
So when working as a private investigator, or on a case - networking is very beneficial.