I get why it exists... to make money.
But it shouldn't. Sizes should be standardised - there should be some guidelines that shops have to stick to. Especially if you're ordering online and the shop doesn't do free returns/refund your postage - you could end up losing a few pounds in delivery charges if something doesn't fit. Personally though, I haven't found my size to change much - I'm a 10 but have to buy between 8 to 12 depending on the style, but generally a 10 is the same in most shops. Also, I don't mind having to take two options into the changing room, it's a bit annoying but even if I knew I was a 10 and it was a set size, clothing styles are different and a certain item might look better being a bit tighter or a bit looser, so I'd want to see options anyway. It does cause problems when ordering online and you can't try things on like that.
Also if sizes is gradually getting bigger, it probably leads to women being in denial about their true size. They could be putting on a bit of weight and going shopping, trying on a 10 and thinking "it fits! I'm obviously still a size 10, so I'm fine" and then relaxing and thinking they must not have put on as much weight as they thought. and so it continues..and eventually what would have been a size 14 nowadays could eventually end up being called a 10 in a few years time. It's like when people justify things by saying, "yeah but the average size in the UK is a 16." Yeah so it's common, but that doesn't make it healthy or ok. We have a growing obesity problem, so yeah, size 16 sounds about right. It proves nothing though.
It's a silly concept anyway that people want to buy a certain size. Girls squeeze into size 8 jeans when they are clearly too small, just because they want to buy an 8. Why?! You are the size you are. The number sewn on a piece of fabric does not change that. Nobody can see the label anyway!