Here's the logic behind using one of these products: The leather on a good pair of boots has the potential to last a lifetime; the stitching, however, will not. Sand, grit, rocksall that stuff can abrade the stitching, leading to the demise of the boots or the need to have them re-built. By protecting those somewhat vulnerable areas with a coat of sealantbasically a form of glueyou can theoretically prolong your boot's life. Years ago we used to religiously slather the welt (where the boot sole meets the leather upper) with a glue-like material that was a precursor to Welt-Seal. The idea here was largely to keep water out, but also to keep dirt from grinding away at the stitching.
Don't rely only on this, though. The general rule of thumb is to try to keep the boots fairly clean (between uses) and keep the leather in good shape with a conditioner or treatment. When the boots are real dirty, wash them with warm water and scrub with a soft brush. Occasionally rinse out the insides to remove sweat residue. Several times a season, apply something like Nikwax Waterproofing Wax ($7.50; www.rei.com) to help seal the boots against water, while also conditioning the leather. Don't use grease- or oil-based treatments; they'll over-soften the leather.