- People like a variety of foods, and
- Brick and mortar restaurants have a tendency to stick to a single menu.
The combination of these two factors means that mobile food trucks have a greater ability to meet a fluctuating market demand for their food.
A taco truck can sit on the same curb for 5 years, or quickly drive a mile up the way to be the only source of mexican food in a given neighborhood. Indeed, they can do both – sticking to a few "set" locations on particular days of the week, and being more flexible, or catering specific events the rest of the time.
This is great because if the local officeworker population knows your truck will only be nearby on Thursdays, then they may well hold off on eating Mexican food the rest of the week, if they really like your truck's tacos. This sort of "predictaburst" traffic also helps in reducing food waste loss – worst case scenario, you won't have enough supplies, and have to close shop early.
And that's just from the "truck perspective" – if as a worker/eater you get a different truck in your office parking lot every day, then you get the ability to eat a wide variety of lunches/snacks without travelling far… Much more so that you would get from a single café nearby. Don't get me wrong, such establishments, done well, are sure to sustain traffic and interest – but even the best restaurants eventually "get old" after 3-4 lunch meetings there a month.
In short, food trucks have greater mobility, that helps them find friendly pockets of consumers; and diners get to mix up their cuisine routine much more than they otherwise would. Everybody wins!