I think this sits in a unique place, as earlier alluded to, similar to Skeleton.
Bootstrap is a large framework for application development, but perhaps you want to hand-roll most of the visual components. Bootstrap isn't very well suited to this, as it is so extensive that building from the ground up (instead of "extending") is a bit difficult to do. Sure, you can modify some of its parts and pieces fairly easily using the variables, etc, but in general, Bootstrap is Bootstrap. Pure, as it mentions on the page, builds on Normalize and gives you some much more basic pieces to work with. I would imagine this would be used by a lot of people who are doing less "app"-like experiences, and are instead leaning more towards layout-driven, brand-heavy "website"-like experiences.
Bootstrap is a large framework for application development, but perhaps you want to hand-roll most of the visual components. Bootstrap isn't very well suited to this, as it is so extensive that building from the ground up (instead of "extending") is a bit difficult to do. Sure, you can modify some of its parts and pieces fairly easily using the variables, etc, but in general, Bootstrap is Bootstrap. Pure, as it mentions on the page, builds on Normalize and gives you some much more basic pieces to work with. I would imagine this would be used by a lot of people who are doing less "app"-like experiences, and are instead leaning more towards layout-driven, brand-heavy "website"-like experiences.