For a little background on what categories are and why they're useful at WikiTree: essentially, categories are groups of profiles, as explained here:
WikiTree Category FAQ
The usefulness of categories comes in when you wish to find profiles that share commonalities, such as where they were born, which cemetery they were buried, schools they attended, occupations they practiced, religions they belonged to, military regiments or medals, special things that set them apart (ie: quintuplets), and so forth. Here's a great overview of how to use these categories at WikiTree:
WikiTree Help: Categorization
So the new widget makes it much easier to use the WikiTree categories--which are accessible any time you are on a profile by looking for a certain icon in the toolbar above the biography section on a profile's edit page (as described in the link to help page above).