Each update is different. There are usually release notes available on the manufacturers website that detail what features or functionalities have been updated/modified.
In most cases, though, the updates are things that shouldn't matter to you. The goal is to improve the product, and 99.9% of the updates do that. There have been cases of errors slipping through and causing problems, but that's the nature of software development.
Being that thing is 5 years old...Denon probably put a note on their website saying there was an update 4 year's ago...and assumed all who were going to find it, found it...and removed it over 3 years ago.Connect it to the internet and let it see if it can find an update. If it does, great.