These brands need protection from counterfeiting because they have built up a great deal of brand equity and counterfeiters can make a lot of money making fakes.
In the fashion industry, a particular label can be more important than any other aspect of a garment. People might think very differently of a particular piece of clothing simply because it has the name of a high profile designer on it. This is because these brands have built up a very solid reputation and people want to be seen wearing their labels.
So, we have a situation where certain brands are valuable and the average consumer could not tell the difference between a real Ralph Lauren and a knockoff. This means that there is a lot of room to make counterfeits. The counterfeiters can make knockoffs that do not cost nearly as much as the real item. They also do not have to pay for marketing or anything else like that. They simply sell the knockoffs as the real thing and make a tremendous amount of profit.
Because this is possible, these big names need counterfeit protection.