For various reasons. Let’s say that you have site with many posts with various tags assign to it. In my example, I have created wordpress theme for cataloging DVD/BD movies. Each post represented one movie, inside content I put synopsis and some movie related data as actors, director…, genre was managing through categories, year and length of movie through post tags. Now, after creating custom post types and transferring posts to CPT, and migrating categories to custom taxonomy with hierarchy, tags was only thing that was left to transfer.