I have been in academic setting for more than 10 years now, but I could not yet understand the urge and importance of attending talks. Theoretically speaking, one would expect to get some insights and possibly learn something. However, if you attend these talks a few times, you would realize that it is a very naive expectation. Furthermore, it is always requested that everybody attend these talks, particularly, if they are presented by someone important from outside of the university or the department, where the talks are usually more scattered.
Let me briefly explain the situation. Usually talks start with something known to everyone even undergrad students, and suddenly in the first few slides it switches to something which is hardly understood even by those who are in the field. Finally, talks end up with something that I could bravely say nobody understand. After a few slides, it is really a mental torture. Making people to listen something annoying is usually utilized for taking confession, which of course, is illegal. One might naively think that it is optional to attend the talk or leave in the middle, but believe me, peer pressure is far more restraining than prison walls.
I was hoping that talks would be categorized into (at least) two groups: educational and research. The purpose of educational talks would be to teach something to the audience, and the job of speaker is to make sure that it is concise and self contained. However, the purpose of research talks would be sharing the new results with the peers in the field, where a small group of audience can discuss the results almost from the beginning without the need for wasting time on lengthy and useless introductions and so on.