So you have a project that will only run/build using an old Node.js version that you can't upgrade but you also do development using the latest version. What do you do? Only build using your CI pipelines? That could work but would make the development process slow.
I've run into this problem a few times, mainly with SharePoint Framework (SPFx) solutions that only support a particular (old) node version. This is where the Node Version Manager comes in and saves us!