Fully taped seams means every single seam on the pants is sealed with waterproof tape, covering all the tiny stitch holes where water could sneak in. It's the most complete seam protection you can get short of premium welded construction.
If you ride in wet coastal climates, spend full days in deep powder, or do any backcountry touring, fully taped seams are absolutely worth it. You'll stay dry through hours of sitting in snow, hiking through slush, or getting pelted by stormy weather. It's the industry standard for quality snow pants and the safe bet for serious riders.
The tradeoff is a bit more weight and cost compared to critically taped seams, which only seal high-exposure areas. But if you're a regular rider dealing with wet conditions, that investment pays off every time you're the one still dry at the end of a storm day.