Probably not all, but a good part of it, for a public presentation, is just giving you a chance to be the center of attention for an hour and to energize students who might be able to attend it. In part it is a "brag" on the part of the faculty about what great students they produce.
There are places, I'm sure, where it is a bit of an ordeal, and it is often scary, but it is also a welcoming you to the core of the profession (for doctoral presentations anyway). Yes, you are good, and we are happy to sit and listen to you for a while as you demonstrate it.
And if it isn't like that for a public presentation, then something is probably broken.
Non-public presentations are more in the line of an exam requirement and you will probably be judged on the quality of your organization and presentation. At the MS level, I would hope that people would realize that you don't yet have a lot of experience doing these things and so expect the student to be a bit halting in delivery, if not organization.
What you need to do is to present the core ideas of the thesis, focusing on insights, and not details. If details are required they will probably come in the question/answer part. Don't be tedious or overly pedantic in your talk.