Their perfect for optin forms and giving away freebies to a large audience very quickly. And since most traffic exchanges allow you to buy credits for around $5 - $10 for 1000 visitors, you can quickly build an optin list or give away your freebie to hundreds of people.
There are also pro traffic exchanges that require a one time join fee - to ensure that everyone that views your ads and websites actually have a working credit card and are not just looking for freebies.
If you can't afford to buy credits you can simply view other members sites and earn credits with just a free account.
There are literally thousands of traffic exchanges and more popping up each day.
Almost all traffic exchanges allow you to sign up new members by promoting a personal referral link.
So you can simply refer new members and whenever they earn credits you usually will be creditted with 10% of their earnings.
Put another way, if you refer just 10 active members and they each earned 100 credits in one week, you would have also earned 100 credits - 10 x 10 = 100.