Do you think a computational math student should go to student seminars about pure math?
You're not going to be able to determine which seminars are useful to you without going to some of them. And yes, some of them will be wastes of time. But I've gone to seminar series that should be extremely topical to me that never benefitted me directly, and some from peripheral departments that turned out to be very useful - as mentioned in one of the comments, even if you don't take away anything from the lecture, it's useful to know people. One day, you might find yourself in need of a graph theorist or some such, and it's helpful to have met them once, or be able to say "I saw your talk..." in an introductory email.

Also, selfishly, from an applied field, having applied participants in more theory heavy fields is helpful for teaching theory students how to present their topics to people outside their narrow subfield.