Why is there a header and logo at the top in a web application?

We are building a webapp, so everyone asks me what content should be in header and why you are using a header at the top.

So my question is why are we using a header and logo at the top, we can use it at the right or left or bottom?

And what content should be in the header?

Answer

The header is usually at the top because it's convention. Why is it convention? Probably because you want to introduce the title and brand of the website you're visiting to the user immediately when the page loads. So you put it at the top. And then it gets out of the way quickly when you scroll down. Sounds like a pretty good solution to me.

Why not put it somewhere else? Well, putting it on the left would put it in the middle of the content. On the right would put it out of the way in a place where your eyes don't necessarily scan immediately (given that most of us read left-to-right). Putting it on the bottom raises the question of "what is the bottom?". The bottom of the window? Or the bottom of the content?

You can put the header anywhere you want, but you should specifically think about why you're putting it wherever you are. Following my first paragraph about convention and use of a header, you should also ask what content to put in it based on that: what's relevant content for initial page load? What should people see before they see anything else? What do your users do with content in the header?