A trademark is your brand. It's how consumers know they are buying a specific product because of the label on the can. But what if you allow others to copy your trademark? Consumers can't tell whose product they are getting (i.e., Coca-Cola soda sitting next to COCA-COLA on the shelves, made by two different companies). Do you think Coca-Cola would stand for this? No. In the short-run it causes loss of sales. In the long run you may lose your trademark because it can no longer be distinguished, and thus distinguish your goods, from those offered by others. To stop this you first have to spot when and where it happens. And that, in a nutshell, is why you monitor for use of similar trademarks.