Affiliate networks handle the affiliate programs for hundreds or even thousands of different companies. In general, they are a little more reliable than in-house affiliate programs so you don't have to worry about non-payment or stats not being correct.
The fact you can earn commissions from so many different services also means that you won't have money left in accounts due to minimum payment thresholds.
ShareASale are an established affiliate network with thousands of companies in their books. They have a solid reputation in the affiliate world and always pay on time.