Concessions are their source of income. Movie companies use their monopoly on film access to demand almost all of the money from ticket sales (something like 90% of the ticket price IIRC), so the theater actually doesn’t make that much off of movies.

Concessions on the other hand, especially stuff that costs pennies to make like soda and popcorn, are pure profit and are basically the only way many theaters can stay in business these days.