You need the plugin whenever you want to serve pages that aren't OMV web interface and you don't want to manually configure files that may be overwritten.
What's the functionality or usage of pugin "openmediavault-nginx 2.4"?
Is to create more nginx instances, like your personal webpage or whatever you need to with nginx.
When do I need this plugin?
When you want to create a website. You cannot touch OMV nginx files mostly because you can end with abroken interface and changes are rewriten on demand, so is pointless
But I don't need to install this plugin for an additional Nginx instance required for Owncloud after installing the relevant owncloud-plugin, right?
No. If you run the owncloud plugin, you do not need to run the nginx plugin additionally.
If you want to serve web pages (besides anything you can find via a OMV plugin) then you can use the nginx plugin.
Please note: Owncloud will be NOT supported any further in OMV 3.0 (which is already beta). But you could consider the docker plugin and use a owncloud docker container instead.
I´d search in those threads:
Docker GUI plugin now stable
and
openmediavault-docker-gui Testing
Just install and try. If you have questions, feel free to ask them in one of those threads 
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