Answer:
they are very effective.
When doing a fundraising, many people like to be able to benefit from their charity work. Having food trucks allow people to feel more cared for and will most likely spend more time at that event or donate more.
People will subconsciously feel more relaxed as it seems that there is something leisure to do during the fundraising event. Having food trucks boosts a person's happiness and thus makes him/her more willing to contribute to the fundraiser.
Also, having food trucks will most likely cause a buzz about your event. People would be curious about the event and you will most definitely get more people to participate.
Thus, food trucks allow you to attract more people and keep them happy, allowing you have a more successful fundraising event.
It reflects psychological context.