Here's a couple of examples ---------------------------------
For example, lets say you have a data set that returned user information. Combining it directly with the html may be easier. The issue happens when you want to use that same data (or portions of a that data) elsewhere. From there you would have to rewrite the query. Having the data-set separated (either in a class or a function that returns raw data, then having that data parsed and placed within html works much better. There you don't have to rewrite anything (and have the satisfaction that the data "works").
On the other side, separating the html will be helpful as well. Say that the interface has now changed. You are not going to remove a column in a table and reorganize other components. Instead of having to look through the code where you may have placed the html with the data, all you need to do is to make your changes accordingly in your presentation layer.
All in all, it may seem like a lot more work to get things started (I don't use MVC in the traditional sense, and instead have my own programming rig). The reward lies when you can separate duties among team members. Someone can work on the presentation (and the browser compatibility issues that it includes) while another one can work on the business logic. So on and so forth.