According to a United Airlines blog post, the company is doing this to promote its new international routes and destinations to Tel Aviv, Israel; Buenos Aires, Argentina; and Tokyo, Japan. It will serve up indigenous desserts from each of those countries as part of the promotion. There's a whole schedule to the various menus they'll be offering from now until mid-August at the New York truck:
From now through July 30 (Sunday): Promoting the airline's new nonstop service from Newark to Tel Aviv, the trucks will be serving rugelach and black and white cookies.
From July 31-August 6: To celebrate the new Newark route to Buenos Aires, the trucks will then switch up the menu to serve dulce de Leche donuts and coconut cream pineapple donuts.
From August 7-August 13: Finally, paying homage to Japan, the trucks will serve cherry donuts and Yuzu lemon zest donuts for one week to tout the Newark-to-Tokyo route.
San Francisco food truck will work a bit differently. The food truck will only tout the route between SF and Munich, Germany, throughout the entire promotion. As such, it will offer free horseshoe almond and apple crumb cakes from now until the promotion expires in August. That still sounds pretty damn tasty.
In New York, the trucks will be serving treats from 11am to 3pm during the week and on weekends. In San Francisco, you can get them from 10am to 4:30pm.