In the era before web apps, the web was a relatively simple system of hyperlinked documents written in HTML. Click a link on a page and your browser would load a new page based on the referenced URL address. The web was brilliantly simple.
However, that simplicity resulted in a number of problems if you wanted to do more than just look at a series of hyper-connected documents. In order to exchange sophisticated transactions with a simple 'fill in the blanks on a form page and submit it,' you'd end up having to reload virtual pages repeatedly.