Things are advertised in order to convince people to buy. Advertisement come in many forms and nowadays, they are not just present in printed materials and televisions but online. I bet, you might have experienced encountering advertisements on ipad 4 cases and ipad mini cases online and you ignore them. These are examples of online advertisements.
Some advertisements are available also through pop-ups on video submission sites such as youtube. There, short videos are shown before the chosen video. These are effective advertisements with clickable links that will direct you to their website such as http://ipadcasefinder.com.
If you are an owner of a certain product company, it is important to advertise them for people to have an idea about them. You will notice that there is an increase in your customers if you will do this because advertisement is the heart of voicing out your products to the world to know.