Inventor Fusion came with the Product Design Suite we purchased. We use Inventor. We do not use Inventor Fusion. Why would someone use Inventor vs Inventor Fusion?
Thats a great question. Inventor Fusion is included in your PDS for a couple of reasons. One is to allow a 'direct' edit of your Inventor models. There is a command in Inventor that opens your model in Fusion so you can make a direct edit that is independent of your parameters. It becomes a chronoligical browser feature that you can edit, etc.
Inventor Fusion itself was a technology preview for years independent of this workflow. It's a direct editing solid model program. You can use it for quick layouts and designs where parameters not only don't matter much, but may get in the way of a quick iteration of an idea. This program has been re-made as Fusion 360, which ironically is now parametric and doesn't share the workflow with Inventor any more. Fusion 360 also is unique in that it's cloud based, free for enthusiasts, and has organic shape creation commands that Inventor Fusion does not have. It's extremely fast at creating designs and great for when Inventor is more than you need.