Manual is typically a on demand state so not running by default, but if something that needs that services runs, it wil often get started. As an example the font cache service, I find this slows a lot of legacy apps so I disable it, but it can also speed up the main windows UI, UAC prompts, Office 365 etc.
If its in disabled state, it never starts, if its in manual state, the first time I do something that benefits from font caching (such as triggering a UAC prompt) it fires up the service.
Off topic note, Build 1809, has very little downside to disable the service in my opinion, but have observed 21H2 has some quite notable performance regressions in WDDM that increases dependency on it when the hardware isnt super fast. One reason I mentioned font cache service as an example as was tinkering with it earlier today.
So the upside of manual is things are less likely to break, as the service if its a dependency will more often than not fire up to ensure such feature works, the downside of it vs auto, is that initial fire up can add delay to whatever task is waiting for it to start and the downside of it vs disabled is you cannot ensure it never runs, in some cases you may want a service 100% off in which case you should disable it.
Thats my 5 pence on the subject.