Undoubtedly, at the PhD level, attending departmental seminars is an important spot in a weekly schedule, where one will get in touch with the latest developments in the field. And usually these seminars are open to undergraduates so one should expect a lot of people when these seminars are department-wide. Also, for cash-strapped undergraduates, it can be tempting to attend events like these in order to eat one’s lunch or dinner (depending on the departments, the seminars can be held at lunch time or at dinner time) there so that they could downgrade their meal plans.
Admittedly, one’s budget can be tight in graduate school, or even in undergrad, so free food can be a good incentive to get students to come. Some depts can be a little stingy and not offer that much food but not everyone will substitute a meal with seminar refreshments so, if you elect to do so, make sure you come early; after all, refreshments are first-come, first-serve. For me to do so I need 10 cookies, as well as a cup of coffee.