Android – Is a system navigation bar required? If not, pros / cons to keep it?

I have not worked on an Android application at one time and I was wondering if the system navigation bar should be there or not.

Navigation bar

Looking at my husband's phone (I'm on iOS), it's not in any of his applications. Is there a new best practice in this regard? I was looking at Google's material design guidelines and it does not say it's a mandate. In addition, the Android Developer's Guide shows ways to hide it.

I posted this on some Slack channels that I'm in and some designers say it's necessary, while others say no, so I'd love to know what you think.

If someone has a note about whether or not to keep an optimal experience, please let me know. Thank you!