StitcherAds allows you to upload the different images, headlines and messages so that Facebook's algorithm can work out which variation gets the most engagement and allocate most on the budget to the winning variation.
The main advert creation page gives the ability to add up to three different images (you can also add up to three messages and link headlines if you wish). StitcherAds then leverages Facebook's algorithm by creating all these variations as adverts in an ad set (find out more about the structure of StitcherAds campaigns here).
StitcherAds does this because Facebook's algorithm optimises adverts in an ad set based on engagement - when your adverts initially go-live they are all shown to users but after each advert has had some exposure, the algorithm calculates which has had the most engagement and essentially gives this advert the lion's share of exposure from the ad set. This is beneficial as you want the most engaging advert to be shown to users.
Additionally, because StitcherAds creates targeting variations at the ad set level, this means that you have your advert creative selected for the most engaging to that specific target audience. You may find different creative is appealing to different target audiences and because of the StitcherAds structure, this advantage is exploited for you.