I use the photoshop symmetry paint tool and pattern preview almost everyday for work as a textile designer and this would be an amazing feature for the ipad app. Using the ipad to draw and create repeating patterns would make the process faster and more organic in my opinion!
Procreate have a symmetry tool that I use all the time and then have to upload to photoshop on my desktop which I then use the pattern preview function to create my final design and at the moment I find I use procreate on the ipad alot more than I use the photoshop app. I think if this function were to be created I'd use it a lot more.