Why go?They are a good way to connect to other people interested in the same stuff. Its good to have those people to bounce ideas off. Meet people to start new projects with.
You learn new stuff! Its way easier learning about something new when you can just sit back, listen, and ask questions. You should be asking yourself, how can I use this, where can I use this, do I want to use this.
The presenter has done the hard work researching what the point of the thing is and how you use it. Sure your not going to become an expert in half an hour. But you will know if its something you want to know more about, and you'll have something to work from.
And usually they give you free food :)
Why present?So they happen! Without presenters meetup's don't happen and no one can benefit from going, the learning, the networking, the free food.
To practise your presentation and teaching skills. A lot of people only do public speaking once in their life, high school speeches. Thats not enough to be good at it. The opportunity/need may pop up someday, and you want to be comfortable to do it well. Sure it may not be a skill you use often but it may be valuable one day. Maybe at a weeding, conference to potential customers / employers.