There are two drivers for this that I've seen. Either bookmarking clothes to build up a look that they will then go and buy, or bookmarking clothes to buy once they go on sale.
One of the things I struggle with buying clothes online is being able to look at all the things I like on the same screen and then decide what I will buy. (H+M does this well). Some retailers limit the baskets to 10 items (Next used to) so even though I'll only buy a few in the end, the only other way to compare items is to open lots of tabs. This doesn't work at all when it's a flash based store. (Esprit.com suffers from this) So I use pinterest for this.
The other time I pin products is when I want comment from other people. I'd rather have these comments on Pinterest than on Facebook.
One thing that would make me pin products more would be if I could make a board private. Then I would pin ideas for Birthday presents and I would have used it to pin ideas for wedding dresses. But private boards are not what they're aiming for and doesn't fit into the context of the article either I guess.