Facebook advertising, Instragram or other social media advertising will reach customers whose demographics, subject area and location match the product or service being advertised. The person who sees the ad will not necessarily make a purchase, but will hear about you or even like/follow your page. However, if it is the case that the customer is looking for exactly what they are looking for at the time, they will make a purchase.
What makes IG/Facebook advertising special?
Increasing brand awareness. Even if the customer doesn't make an instant purchase, they will have heard of you. When looking for this particular product or service in the future, the person will usually choose from well-known/heard brands.
Gathering and working with the target audience. If a potential customer likes your page, there is a good chance that they will make a purchase in the future. They will be constantly shown your promotions, newsletters and announcements.
Making the product/service known to the public. Maybe customers don't even know that the service or product exists? You have the opportunity to inform and interest them.