The second major reason is the interaction with others who are your peers, or will be your peers. Presenting a talk at a physics conferences is one way for you to get people in your field to recognize you. Chances are, when you are done with your studies, these might be the very same people who might be the ones you seek employment from. It is never too early to make a name for yourself, and for people to start recognizing your name and your face (not to mention, your ability). Do not underestimate the importance of making contacts.
This wonderful article from Science Careers expanded more on many intangibles surrounding the hiring of scientists that involved more than just hiring someone smart. I think anyone who's about to go into the workforce should pay close attention to what has been written here.
Some of the managers I talked to believe many young scientists acquire this worldview during their training. They are, in the words of one of my clients, caught up in the "perfect world syndrome." Scientists with this ailment have trouble relating to the way things work in thenonperfect world we live and work in. Although most scientists recognize that there is more to success, in the corporate world and in life, than brainpower and purely intellectual feats, many of those same scientists fail to fully recognize the implications. They get upset when people don't hire them just because they're smart.
It's important to realize that companiesdosearch for the best, the very smartest people they can find. There isn't a hiring manager in the world who'd admit to anything less. Every search I've conducted over the past couple of decades bears this out. Except that those people--the people who get hired--have other virtues that are equally important in the workplace.
A busy manager can't be so shortsighted as to look for only smarts. She has to look for a person who can do today's job--plus what the position will morph into in the near future. In addition to that, she's hiring someone who needs to work well with a variety of people throughout the organization, so personality counts as well. That's bound to upset you if you are the "perfect world" sort.
There's the problem in a nutshell. The world of employment differs from this view of perfection because it takes more than just brains to make an imperfect world more perfect.