There are a few main reasons behind the popularity of brick as a material, with brick slips being the best way to use it.
Colour
The natural colours that come from dug and fired clay resonate with people, but what's more effective is the natural variation that comes with the process.
Texture
Even with no creasing or tumbling on the bricks, the wall (or floor) will have the texture of 10mm mortar joints. What makes brick slips really stand out though is the texture on the brick- from misshapen tumbled reclaimed style products, to the gentle creasing of traditional style ones, it makes an incredible impact.
Timelessness
By using a product that is all around us, and has been for centuries, you guard yourself somewhat against changing trends. Particularly where you're using a reclaimed style brick slip, it has the look of already being there for years as a beautiful fixture.
All 3 of these can be summarised simply as character- you can add incredible character to even the boxiest of new builds using brick.