PresetMagician loads VST plugins in separate processes (currently named
PresetMagician.RemoteVstHost.exe) to ensure that a crashing plugin doesn't crash PresetMagician. Think of it as a certain number of mini-DAWs running in the background.
As opposed to your typical DAW workflow, where you would only load a few VST plugins per session, it's a common use case with PresetMagician to load many VST plugins per session, especially if you just started using PresetMagician and you're analyzing 100s of plugins.
The VST Worker pool is a pool of VST hosts to ensure that if one VST host crashes due to a plugin, the next one can quickly be used. Also, some plugins don't properly unload, keeping memory from being freed. To counteract on this, PresetMagician shuts down a VST host after a certain number of processed plugins to free up used memory.
Also, the VST Worker pool allows for for a future feature where plugins could be analyzed and presets exported in parallel.