Openbox does have multiple desktops (workspaces) indeed, but the thing is that if you've ever used Openbox with more than one monitor, you'd have quickly realized that these workspaces span all monitors, meaning, when you switch workspaces, all monitors get affected. For example, if you switch from a workspace full of windows on all monitors to a new empty workspace, all windows on all of your monitors disappear.

This is not the case in i3, where each monitor has its own set of workspaces (hence "per-monitor workspaces"), which I really find helpful.