I've done some event planning in the past, and it is a high stress job. You have to count on vendors to do their job, and do it well.
Don't assume they want a kickback or for you to buy onto their preferred list. Some may do that, but the best make their money off the success of their events, which means that their preferred list is made up of people that they can count on to make them look good.
If they recommend you, and you fark up, they lose credibility. If you do a great job, they look good.
Catering, like a lot of creative businesses, is all about building relationships and trust.