Seat selection is a big bonus, i.e. bagging exit row on European economy flights and as someone further up said on the quieter flights a higher chance of an empty seat next to you.

Lounges are not the height of luxury, though the T3 Cathay business lounge is well above the usual standards and almost always quieter than the BA lounge. I work from home most of the time and given wifi can happily work in a lounge for a few hours. While lounges can get busy they are a lot quieter than terminal seating, and I know T5 south can get busy at times. In Europe the food choice in lounges is a lot less, though there are some that are better e.g. Helsinki

The avios bonus helps to build up the stock for upgrades.