The way that you can use this tool is actually quite simple. You can basically use the cloud vision tool and its “drag and drop” image feature to find out what Google in particular is going to feel is a relevant interpretation for the image that you are currently looking into.
This can help you shape the direction in which you might end up taking your SEO all in all, which is something that you are certainly going to find very useful indeed if you think about it.
However, you should bear in mind the fact that using this tool is probably not going to help you better understand how the algorithm works since Google’s tools are notorious for not shedding enough light on this sort of thing. It’s only going to give you a better understanding of your own image’s relevance.