It saves your workspace layout so that the vault will reopen in the same state that you left it in. The contents of workspace.json in one of my vaults:
- What notes are open in what tab or tab groups and the layout of windows, sidebars, etc.
- What items are in your ribbon
- Last opened files
If you don’t want to sync it, that should be fine, but the state of the above won’t be the same between the desktops.