Friends just booked a flight to Venice via Paris, for travel in February and they found that if they had just booked Atlanta to Paris and not be going onto Venice the fare would have been over $100 more expensive

and whilst this makes NO sense to me the Point is:
It may pay to ask about all destinations if you just want to get to Europe or you can re-arrange your itinerary a bit

This makes sense for airlines. It is estimated that it adds 0.5-1% to the revenue by doing the network management.

If one plans to travel to several destination spread geographically, it pays to do many what-ifs to compare different entry and departure city combination as well as the days and hours of the departure.

That's very typical. A nonstop flight to Paris usually comes at a premium, compared to a flight to Venice that involves a connection. So even though the mileage is greater in the second case, you pay for the convenience in the first case.

And a point should be made that if you book say a USA - Venice ticket via Paris you cannot get off in Paris without paying a huge penalty - so if booking Venice then you must go to Venice - airlines are not that dumb to let you abscound off the plane in Paris.

And one should keep in mind that airline's take steps to prevent someone buying a say Atl- Venice ticket via a change of planes in Paris from getting off in Paris - in that case you will have to pay a penalty of perhaps hundreds of bucks - not sure how they can do this but they do

The going rate for roundtrip flights MUC or FRA to EWR in Jan and Feb is about 170€ right now. Not bad, but who knows if Continental Airlines will still be there 3 month from now.

So if you get off in Paris and do not take the connecting flight, they will charge you? I thought thye cancelled the return ticket (assuming you are from the US) All carriers - I assume the OP is on Delta flying from Atl.

Non direct flights are nearly always cheaper than direct flights. So changing planes in Paris drops the fare.

suec1 - yes you are right and i should have thought of that - i guess that is penalty enough to prevent that

Airline Revenue Management 101 is in the core curriculum at Hogwart's.