“On Facebook ads, what if I didn’t run my ads using interest. Will it affect anything?”
Not adding interests to targeting doesn’t affect how the ad run, but it will affect how well your ad performs, especially early in the campaign. The interest (and demographic) data help Facebook show the ads to the correct audience sooner.
It’s possible, especially with a conversion campaign to run ads with little to no targeting and get results. This type of targeting takes advantage of the Facebook algorithms to match content to who it thinks are interested parties.
As of June 2017, I haven’t found that a “USA all”, targeting in conversion campaign performs better than using interests to get started. It is possible to some extent, but for most small businesses, the cost to get to optimization this way will be too expensive.
Where targeting without interests makes more sense is when you have a “Lookalike” audience of people who have done something meaningful to you. With lookalike audience, for instance, you can upload a list of your customers and Facebook will build a list of around 2 million people (1% USA population) that have similar characteristics to the audience you uploaded.
With this audience, I lean towards not using any interests. Facebook has already determined that these people look enough like the original to be in the audience. In some cases, I will add demographic data if these are specific attributes that a customer need to have.
As time goes on, I believe that the Artificial Intelligence that Facebook runs on will afford a much-simplified targeting interface. Eventually, we will be focused on the results of the ad, and Facebook will find the best audience to produce the results. But we aren’t there yet.