It's an insurance policy.
Just in case your fermentation didn't finish and there are viable yeast (or mold) still floating around, the sorbate would inhibit the restart of fermentation and (theoretically) corks popping down the road as the pressure built up.
I added it to my first 4 kits (2 reds and 2 whites) and have not added it since. I also added it to my Viognier as it had a reserve pack with sugar.
I don't think you can taste in in reds so much as in whites or at least that has been my experience to date.