It cuts out about 3 steps.
For example, say I wanted to use the "you just went full x you never go full x" template (which isn't one of the Popular Memes) on a meme comment, but I don't have that picture. I'd have to search for it, right-click and save it, then upload it on the Meme Generator. THEN I could use it for the meme comment from My Templates.
The problem that can arise from this is that I probably will never again use that image, or very rarely so, but it will replace the oldest template in My Templates, which I may actually use all the time and will have to re-upload.
If "single black line" is in your My Templates already, you just use it and click Add Image, making ALL templates on Imgflip more readily available for meme comments.