I would say this is one of my most important tools in my long list of SEO tools I have purchased.
I know I can use Google search console for free but having this tool is a no brainer if you simply want to have a quick snapshot of all of your websites to see what pages are indexed. This has saved me a lot of time. The user interface is easy and great to use. I highly recommend URL monitor if your on the fence to give it a go.