Face and Chase so that Parts go into Place.

Whenever you get a new frame straight from the factory, you have to put it through a machining process before it's OK to add components. 

The tubes that make up a metal bike frame are never perfectly round, and even if they were, the heat from welding or brazing processes warp them into ovals. The most expensive tools in any bike shop are the facing and chasing tools that cut away metal to correct the mutating effects of frame building. The facing tool is a cutter that makes the top of the tube flat, and the inside of the tube round. The chasing tool cleans the threads of the bottom bracket. This final bit of frame preparation is usually not done in the factory because it's time consuming, expensive, and strips away the paint. Frames with disc brakes usually need to have the face of the brake mount machined too. 

Failing to face the tubes and chase the threads will make it so the components won't fit in properly, which leads to a gang of problems ranging from creaking to premature failure. 

Below are some snaps from a frame whose headtube I recently had to face. 

This is the facing device. Those two cutting wheels do the hard work, the plug at the top keeps tension, and the handle is for spinning. 

Normally this tool would be covered in cutting oil. I wiped it away to show the metal flashing and paint that the cutting wheels strip away. 

This is the tube before cutting.

Here's the tube after a few spins with the facer. Note how part of the inside is bare metal, but in the center of the shot there's still paint. That's because the tube is slightly ovalized. Once I'm done the headset should press right in without issue, and will be able to stay in there for years without creaking or failing. 

Headtubes always need to be faced. Bottom bracket shells always need to have their threads chased, but only need facing if you're using an outboard bearing bottom bracket, like a SRAM GXP, FSA Mega Exo, Campagnolo Power Torque, and so on.