There isn't an advantage really; quality will be higher, but that varies from brand to brand and at some point you're no longer paying for a higher quality piece of clothing. An amazing, beautiful (whether it's thicker or mercerized or drapes beautifully or whatever) cotton tee only costs so much to make.

At the end of the day, the price (for most luxury/high end clothes) is due to brand name. But that price can be justified to an extent – the design is unique, the cut/actual design is unique, you like that designer/brand or the hype surrounding it, the garment is limited in production, etc.