You get the opportunity to find out what the product or ad is about. If you are talking about a product, the first part is usually the positive side of the product. Then comes the negatives, which are usually at the end and read very fast. Although if you're talking about a show, they usually just show you a trailer of the movie or show. If you are talking about ad that talk about people, such as those election ads you see a lot now a days, then they usually talk about the negative things about the person. But if they are talking about themselves, they just say all the positive things about themselves.
By identifying the purpose of an ad, you are able to see what it is actually about. If you didn't know the reason behind the purpose, you might be confused as to why is this ad even an ad overall. Whenever you find out the suitable audience the ad is directed to, you are able to see if it's suitable for you. For example, if there was an ad featuring a men spray a female would most likely not buy it but a man would. This is the same for the other gender. Of course if some females like men spray, they would buy it. However, you can clearly see that, that ad is forwarded towards men.
