I guess you could say its because you might have an opportunity to get help off each other. Maybe your area of design could help another designer who is weaker in that particular style. Or maybe one designer is aware of a job that another designer would be suitable for and could pass on your name?
I think its good to network with anybody really, sometimes hearing what someone else is doing can give you a kick up the a**e and we can all benefit from bouncing ideas off each other.
@Rossm Would that main point then be to mainly work together? (or at least to have that possibility) I don't know about you, but if I am designing something for my client, the last thing I want is work with another 'networked' designer who might get the job rather than me- if you get my drift.
basically freelance networking is good for 2 reasons.
1) helping each other out when you have extra work
2) being able to supply contact details or work which you don't do, for example do 3d but my client wants an illustration.
When you get to the point where you have more clients than you can cope with you get to choose, to some degree, which clients you want to work with and which ones you want to turn away. See, if you do that it's unlikely that they will contact you again for another job as you bluntly turned them down. If you can tell them that either you're too busy but you know someone who can do an excellent job for them or you accept the job and outsource it to someone else then you've still helped that person with their original request. They are much more likely to come back to you in the future with potentially more valuable work or refer you to other clients who again may provide more valuable work.
And again, to expand on Levi's second point, if you send one of your friends some work then this is you doing them a favour and unless they're an arse hole then you can expect them to return that favour one day, perhaps when you need it most.