The taping of seams is necessary in waterproof clothing because needle stitching leaves holes in the garment through which water can seep in when you’re out in wet conditions. Tape made of strong nylon cloth with a very thin yet heavy-duty waterproof rubber backing is used to seal off the seams of the garment, to make the stitching between the arms and torso completely watertight.
Seam tape can’t be applied at home by yourself so it’s common practice to look for waterproof jackets, trousers and tents with the taped seams feature to ensure they’re fully waterproof. Manufacturers use set machines to melt the tape over the seams, without melting the fabric in the process.
Apart from being a quality guarantee, taped seams are a way of ascertaining waterproofness in garments and strengthening the material in general.