Some of the universities our common app list indicate that the personal essay is “optional”.
And should the student just skip the essay submission in these cases ?
They will read it if you submit it.
If you feel the essay is a good reflection on yourself and adds to your application (as one hopes it will be) would recommend submitting.
IMO “optional” isn’t really optional.
If you have the essay, use it. If you don’t, depending on the school, you can likely skip - for example a high acceptance public.
I think some on common app say it so you don’t feel it’s wasted but likely has little to no impact. For more competitive schools, maybe more impact.
The optional part is a way for them to secure more apps.
If you are already submitting your application to that school through the common app, adding an additional “optional” essay is not “adding another application” . A personal essay gives an opportunity to let them know a little bit more about you. It’s like being able to tell someone enough about you that they might want to be your roommate 
Also, how would you feel if you/your child were denied and you have no idea whether if they had made the extra effort to write the optional essay, it might have made a difference. I would strongly recommend that you consider “optional” not “optional”. Put the extra effort in to do it, unless you truly, honestly do not have the time to do so, because if others don’t and you do that speaks highly of you.
They will read if you provide. As this is the part of the application that turns all those stats into a real human being, it’s best not to skip it. It is generally additive. In all likelihood, you have one for other schools.