@fronflower already answered the technical side of things, but I'll give some personal examples of why I use Pro. I began my GIS journey in ArcMap, so adjusting to Pro has been... A challenge. But I've made a lot of headway in the last year!
Here are my main "pros" of Pro:
- You can connect directly to Portal and easily use/update layers from your account as well as your organization. This has saved me a TON of time.
- You can build multiple layouts and maps in the same project.
- By building everything in a project, you can easily share your work with others.
- Many processes that took forever in ArcMap are performed quickly in Pro.
- You can Analyze your tools before running them to check for errors.
