You use no parametric-history based CAD product because it looks intimidating.
You don't want a massive learning curve learning stuff that seems stupid to you.
You realize that you don't want to waste hours using a rollback bar to try and understand some pick your nose engineer's design intent just so you can modify their solid model to machine it.
You have to work with other people solid models.
You have to work with non-native solid models.
You use Mastercam and realize you want more powerful CAD.
You have seen SolidWorks and don't wish to be a CAD expert.
You use SolidWorks and realize it leaves a lot to be desired.
You use Pro/E and realize it leaves a lot to be desired.
You use Surfcam and realize Surfcam CAD leaves a lot to be desired.
You use Featurecam and realize Featurecam CAD leaves a lot to be desired.
You use Rhino and want a modeler that's much better for mechanical modeling.
You use Mecsoft Visual Mill 6 and want a better CAD program.
You use an old version of BobCRAP, hang up on their telemarketers and refuse to downgrade your old copy of BobCRAP to BobCRAP 2007 / V22 and give these lying Scientologist slime balls another dime.
I have some good news for you:
I can show you a better way to model!
If you are in the following categories you're a brain dead idiot and I can't help you:
You use AutoCAD and think 2D is good enough.
You use Gibbscam and think it's CAD and CAM are just fine and have no clue that Mastercam X2 blows Gibbscam away in most areas.
You use BobCRAP 2007 / V22.
You use Alibre.
You think Matt Lombard is a genius and actually knows something about manufacturing.
You think Franco Folini is a genius and actually knows something about manufacturing.
You think Novedge actually adds value to the CADCAM products they sell.
You think that CATIA is an affordable answer.
You think CATIA has the equivalent of what Solid Edge with Synchronous Technology has.
You think SolidWorks has the equivalent of what Solid Edge with Synchronous Technology has.
The Jon Banquer blog has linked to a new video which makes it very clear how much better thought out Solid Edge with Synchronous Technology is and how easier it is to model with their new tools.