how to start/stop/restart processes
I'm using MAC much years, and never needed to use the launchctl command. If mean never, mean in normal usage, of course sometimes needed use it when installing somethings from the macports - for onetime load of startup files or so.
I'm wondering, why you need start/stop/restart services with launchctl? Asking because if you not an experienced OS X users you probably will screw the system &quot;harmony&quot; :). So, in this case - IMHO - better to ask the specific problem, so ask how to start/stop the specific service.

I don't see a problem with asking the wider question as to how services are started and stopped if it enables the OP to better understand the underlying mechanics of the system.
As to why you would want to do this, one good reason would be to remotely kill errant processes on remote machines via Apple Remote Desktop. I do this frequently for our network users at school and it can save a lot of leg work (as well as avoiding entering repetitive terminal commands).
ARD users will want to kill processes by name rather than by PID as the process ID isn't necessarily the same each time the process is initiated...
sudo killall coreaudiod

...would be a useful command for ARD users as it's repeatable regardless of PID, whereas...
sudo kill -1 211

...would very likely only work to kill coreaudiod on the occasion that its PID was actually 211, and definitely wouldn't be an ARD command worth saving for future use. This may seem obvious to &quot;advanced&quot; users but unfamiliar users may not realise the difference.