Alas, I am not able to recognize the usefulness of the starter for picom in the submenu Accessories. 
Sorry for asking a perhaps silly question, but shouldn’t the existing starter for its configuration in the submenu Preferences be enough?
Thanks for any ideas.
You can see where this comes from: it’s the setting of Categories=Utility; in /usr/share/applications/picom.desktop, as provided by the picom package and ultimately stemming from the upstream code.
Looking at the Freedesktop specification for categories I’m inclined to believe it’s better set as System (‘System application, “System Tools” such as say a log viewer or network monitor’) than Utility (‘Small utility application, “Accessories”’).
To that end, I would recommend that exact suggestion to the upstream developer.
I should add that the other picom thing you see is not for picom itself, but is for picom-conf, the configuration utility.
Thank you, wxl, for your detailed and quick answer!
Now I see that I formulated my question in a misleading way. The point is, because it is autostarted, why is a starter for picom necessary at all? It’s my impression that users seem to need the existing starter of its configuration utility only. So what do I fail to notice?
(I understand that the starter is created by the developers of picom, hence the Lubuntu team is not responsible for it.)
Remember we’re dealing here with the picom package which should ultimately be universally applicable to anyone using the Ubuntu repositories, regardless of their flavor or configuration. Consider the most minimal of such situations, where a basic core system is installed but no desktop environment. I’ve seen some really minimal set ups where a typical autostart setup (as defined by Freedesktop) isn’t even necessarily a reasonable expectation. But even if you were to use Openbox by itself (meaning no LXQt or other desktop environment around it), you’d still have menus. This would allow you an easy graphical way to start picom. So to cover all the possibilities, it does not make sense to remove it.
That said, there is a potential argument to me made to add another desktop file that hides it and include that in the lubuntu-default-settings package (e.g. like we do with /usr/share/Lubuntu/applications/lxqt-hibernate.desktop). However, there really is no negative effect to having it there. If you click on it, it doesn’t do anything. Run picom in a terminal and you’ll see why. tl;dr you can’t run more than one compositor at a time.
I for one like it there, because I only use picom for movies ( picom gets rid of screen tearing when using intel under modesetting).
I prefer having it default to off (Session Settings - modules or autostart) when doing anything else, because it’s one less thing running in the background.
Thank you for all your kind explanations!
Indeed, for me the starter seems to be needless and therefore it got deleted.