Fitts’ Law is an interesting attempt to logically ‘prove’ usability. As described here, it is simply this:
“The bigger an object and the closer it is to us, the easier it is to move to.”
For web design, it means grouping similar options, and reducing the space in between. It seems intuitive, but there is a great deal of evidence to prove its worth as an idea.
However, it’s possible to go overboard with this approach. There are diminishing returns on making buttons bigger, so at some point, there is nothing to be gained by making something larger.
Also, as discussed here, sometimes you don’t want something to be easily clickable, such as ‘delete all’ buttons. Depending on the impacts to the user, Fitts’ Law can be disregarded.
So Fitts’ Law is interesting and useful to bear in mind, but it’s also useful to know when not to apply it.