It allows you to use separate controller functions, one for post one for get, which stops repetitive if (isset($_POST['param']))
to check when you should and shouldn't be attempting to access the POSTed data and such.
It also allows you to limit access to routes depending upon the method used to access them. For example I like to make all my ajax routes only available via. POST.
You can use Route::any() to cover all methods if you don't want to separate access methods.
On a site note, no idea if this has anything to do with why it was developed this way, but when being strict about unit testing, functions should be kept as short as possible, to a maximum of 10 lines in order to help keep code easily debug-able. Separating the access methods allows you to reduce the amount of lines within controller functions.