The plugin_host application in Sublime Text 3 is eating ram , what's its purpose?
Starting with Sublime Text 3, the embedded Python environment that is used to execute plugin code was moved from the core application into an external process named plugin_host.
This is a "safety" mechanism in that if a plugin does something to cause the Python interpreter to crash (say by incorrectly using the ctypes module or other native code), in Sublime Text 2 that crash would bring down Sublime as a whole, while in Sublime Text 3 and above it crashes just the plugin_host, leaving your file content intact so that you can save your work and do an orderly shutdown.
Thus, if the plugin host is consuming a lot of ram, the reason is that there's a package that you've installed that's causing it to happen. You may be tempted to kill the plugin host, but since that stops all plugins from being able to run, and a lot of the core functionality of Sublime is implemented via plugins, your editing situation will be less than stellar.
Your better bet is to diagnose the issue to determine what package is consuming that much ram and determine if it's worth still using or not.
As a side note, in Sublime Text 4 there are now multiple plugin hosts, a legacy one for older packages and a newer one running Python 3.8 for newer packages to opt into. Thus depending on what build you're running, you may see more than one plugin host running.