update-desktop-database only updates the mime types handled by your applications. Nothing else. What changes do you expect to see? For changes, you may have to log out and log back in, or even restart.
If I add some Actions= then I would like to see them when I right click on the particular application. I mean if I make any change in the .desktop file , I want a command that will absorb the changes and reflect on my machine.
For example, if I make any change in /usr/share/applications, it immediately shows. This is not true for ~/.local/share/applications/, at least in my machine.