Running a successful (or not so much) business supposes you to know your customers. And one day you have to move on, find fresh blood for your brainchild. You need to find those interested in your product.
To help advertisers avoiding a manual addition of interests, Facebook offers an Interest Targeting Expansion. Users of interest-based targeting have probably noticed the checkbox to “expand interests when it may improve performance at a lower cost per result”.
So, Facebook automatically expands the interests if finds it worth. But is it possible that a machine knows better than you about who should see your ad and do you have to actually use it? You will be surprised, but everything is going to the audience's size again.
Using the Expansion for small audiences (less than 100 000) decreases the cost by 20-30%. For bigger audiences (about a 1 000 000) Expansion raises the cost. Smallest audiences accounting less than 100 000 make it too difficult for Facebook to optimize well "comme il faut" and find the necessary amount of right people. It ends up in higher CPM and lower conversion rate. So, it looks more than worthy to use Facebook Interest expansion. It will turn your for example 50k audience into a 1 million one, still specified.
If you work with quite big audiences around 1-2 millions, potentially you won't have any benefit of using the expansion, at least while your CPA doesn't rise.