Tack free time is important on certain systems, because you can be using them at a job site where you have a lot of sawing and ground movement with excavation. There can be dirt or contamination in the air and you want to be able to try and avoid having that contamination adhere to the sealant surface.

Having a short tack or open time is important in order to get the material to skin over and be resistant to any type of airborne contamination.