Can anyone explain the difference/reason why there is Routing and Direct Routing in the mixer?
Direct Routing allows you to have to have 7 post fader / post panner Routing destinations.
I see, the difference ist that Direct Routing offers 7 destinations “in parallel” whereas Routing does only allow one destination to be selected.
This probably describes it best in use:-
The summing mode is also an important factor in how the processing works.
When in summing mode - yes. But it also let´s you switch (and automate!) output routings in Standard mode for example.
Will that be any different from sending to multiple FX tracks?
It is much faster than stting up 7 sends (my opinion), and it is an output routing, not a send. With the send you still have the original dry signal. Of course you can tweak both to behave more or less the same if you want to.