According to Kegworks, the answer might be "all the time," particularly when considering Czech-style pilsners.
In discussing the side pull beer faucet, Kegworks describes its features and uses:
"This combination of features allows you to create thick, dense heads of foam with the side pull faucet that is part of the ritual for consuming Czech pilsners, including Pilsner Urquell. This foam, however, is not the dreaded beer foam you get from a typical draft faucet. It is a wet foam with a smooth and creamy texture packed with flavor and aroma from the hops. It lasts longer than dry foam and is meant to be savored along with the liquid beer in your mug."
[Emphasis mine.]
Wet foam is made by submerging the side pull faucet into the beer while pouring - usually verboten - and is especially desirable when pouring Pilsner Urquell. The amount of foam varies from zero foam to 100% foam. Again, from Kegworks:
A Čochtan pour is just a mug filled with beer and no foam. Without the foam, you lose the balanced flavor, and thus, you get a very bitter beer.
Mlíko, or a milk pour, is a mug full of sweet and creamy wet foam that really shows off the Saaz hops and aromatics in a Czech Pilsner. This is traditionally enjoyed as a dessert beer that you sip quickly.
Check out this video to see examples:
The idea of an entirely foamy beer runs contrary to my entire beer upbringing. Once I discovered craft beer, I learned a foam head on top of a glass of beer was a good thing. Now, I'm learning that it's a dreaded thing! Which is it?
I guess it depends on what kind of beer you're drinking but I'll be curious to try a "correctly" poured Pilsner Urquell sometime, especially if I can try it side-by-side with a improperly poured one.
Not to knock Pilsner Urquell, but I wonder if a wet foam would improve the taste of a Michelob Ultra or Bud Light. Back in my college days, beer foam was indeed dreaded. It simply meant you had less beer in your Red Solo Cup. Could it be that it would have been a flavor enhancer for all that cheap beer?
Worth trying? Maybe. But here's another thing I'm thinking about trying: the Beertini (olives or pickles added to beer).
Whoa!
#BeerCareful