Service-Engine-Host - which is superficial?

[Torsten Schlabach <ts...@gmx.net>]

Dear list,
 
I am trying to set up a somewhat complex virtual hosting with Apache and
Tomcat.
 
Having done some experiments, I wonder which setup I should go for.
 
What I don't understand is:
 
I can have several services, engines and hosts nested.
 
I can see what different hosts within one engine could do for me, such
as having a separate webapps directory for each virtual host.
 
But in what situation would I want to have more than one engine or more
than one service?
 
Torsten