Is the pouring method of beer really important? Beer afficionados and similar seem to adhere to some method of pouring beer. The reasons stated feel unscientific, and I'm wondering if there's any truth to it?

What I'm talking about is the method explained e g here: https://www.beeradvocate.com/beer/101/pour/


  Steps to a Perfect Pint
  
  Use a clean glass. A dirty glass, containing oils, dirt or residuals from a previous beer, may inhibit head creation and
  flavours.
  
  Hold your glass at a 45° angle. Pour the beer, targeting the middle of the slope of the glass. Don't be afraid to pour hard or add
  some air between the bottle and glass.
  
  At the half-way point bring the glass at a 90° angle and continue to pour in the middle of the glass. This will induce the perfect foam
  head. And remember, having a head on a beer is a good thing. It
  releases the beer's aromatics [...]


(emphasis mine) 

Another common explanation is that a foam head actually works as a "lid" and thus keeps the beer carbonated bettter.

Is there any truth to either of these?
The release of aromatics is most certainly part of the reason for producing a head when pouring beer. You're creating a buildup of bubbles that contain carbonated gas coming out of solution; that gas can carry aromatic particles created during the brewing process. The more there are, the more aroma is released.

The "cap" theory is pretty dubious, on the other hand. The head isn't solid, it's just composed of gas bubbles that can be pushed aside by other bubbles as they escape out of solution.

Another reason not mentioned is that pouring a head allows one to carry a full glass more easily without beer splashing out. The bubbles in the head keep the liquid level below the edge of the glass, and they also provide a cushioning effect as the glass is moved around. It's odd, but from working behind a bar I can tell you that glasses with a decent head are easier to transport without spilling.

The specific pouring method described works consistently with most any beer, thanks to a moderate amount of contact with nucleation sites, which give the carbonated gas dissolved in the liquid somewhere to "grab" and form bubbles. Pouring the beer down the side of the glass provides contact and some (but not too much) agitation; straightening the glass then prevents any accidental spills as the level rises. Again from time behind the bar, I can tell you that this takes a little bit of feel and practice to do smoothly, but it's by no means difficult. You'll have to pour a few beers to try it for yourself.