I just got an ST2 and for some reason it runs a 170 rear tire. If the only difference in the ST4 is engine, why not the 180? Would that be an upgrade to switch? Just curious as to why they spec it out as 170.
Wider tires are more and more common and more fashionable. They can serve some purpose, but often is cosmetic as we see in many "choppers" and cruisers of today. If 3 cookies are good, 13 are "better."
As has been said, you won't see a any real difference. Real differences come in the cross section and compounding. Years ago I went from the old "Conti Twins", the "go to" tires of the late 70s and early 80s, to a set of Avons. The Avons were not only wider, but had a flatter cross section and with modern compounds (of the day), more traction with better wear characteristics. The improvement in handling was dramatic. However, this was a pair of tires that were designed to "work together" in a modern sense, not a marketing sense, or to be honest, the understanding of a prior generation of thought.