Interest-based targeting is generally used for cold audiences – when you want to introduce yourself to people who haven’t heard of you before.
When you search on Google, you have intent. You know what you’re searching for – you have an intention of finding something. On Facebook people are looking to see what’s up with their friends, be inspired, or get new inputs – but they don’t know what that’s going to be. They have zero interest in you or your ad. Zero.
It’s up to the advertiser to create that interest, and one of the sneaky ways to do that is by using what we already know they are interested in – the stuff they put on their Facebook profile themselves or third-party sites that Facebook has tracked them on.
The biggest advantage that interested-based targeting offers you is that you can speak to what you already know people like.