Why would someone want to wear seamless underwear? Some men find them more comfortable than the regular kind. In these kinds of undergarments, there is no sewn edge on the inside. This means no loose threads or raised border to irritate you, itch you, or just plain bother you. It may also make the underwear threads less likely to fray. And aesthetically speaking, the overall look is slicker and more uniform.
Here are a couple of examples I like, both from Jockey:
Seamless underwear for men pictured:
Left: Jockey Men’s Underwear Seamfree Low-Rise Brief
Right: Jockey Men’s Underwear Go Seamless Waistband Boxer Brief – 2 pack
Seams or not, another important factor in how comfortable any garment is, is fabric.