Let's say you have a mobile phone you would like to sell. You might post a for sale ad on a Classified Ad Website. Many people go to a classified ad website when they are looking to buy or rent something from an individual seller/renter.
EXAMPLE 2: SOCIAL MEDIA MARKETING
There is no avoiding a platform with 2 billion potential consumers of your product. Posting an ad on social media could promote it to more people than any other way.