It's not just Intamin that do them, but, a lot of the time the reason they're done is for the clearance envelope when the supports/theming structures come extremely close to the track/ride envelope.
To add to this - I think there's a whole bunch of reasons that they do pull-throughs. Although almost all of this is based off educated guesses.
1. Envelope/clearance testing.
2. Chassis testing - To make sure everything has been assembled properly in terms of track joints and stuff. This might be even more important with a new train design.
3. Local regulations - This might be the reason we don't see clearance tests as often. Maybe there's something locally that means they're required to do the tests.
They may appear to be going slowly,
@ATI, and you compare their progress to Kingda Ka, but with KK they do only have to winch up and over the top. With this, they've got to move an awful lot more distance along the ground, and cover much more track length.