I will try again.
First, I made fun of Josh for his when vs. when mixup... Now that's wasted.
People add LME/DME when:
- increasing mouthfeel and ABV has you noted. Everyone but Josh gets 3.1% out of a can of the base extracts (he gets 3.7%, but he is special). Many like beers over 4%. That's why Mr. Beer defaulted a while back to including booster with the base refills. Me - I'd never use booster again, substitute a packet of LME/DME instead. Much better.
- They want to impact FLAVOR. Go read the descriptions of the 4 different LME/DMEs that Mr. Beer offers. If you have a bland refill and what to make it more ROBUST, add the Robust LME/DME. Or if there is some flavor in the description that you want to accentuate.
- Improving head retention (or having any). Add a packet of Golden (wheat) to your brew and you will see a nice head. Or, steep 4 oz of Carafoam or Carapils.
When I brew Mr. Beer refills, I add a pound of LME (I buy it bulk) to all of them. I like MALTY brews, and don't care for pilsners and such. More malt is always good.
Many follow this progression:
- Brew base refills.
- Brew Craft refills and Seasonal refills (a lot more malt in them)
- Do recipes that add a dry hop.
- Add steeping grains.
- Add LME/DME and boil hops in it to add some bittering.
- Drop HMEs in favor of steeping grains combined with LME/DME and hop additions, making recipes they find on the web or create themselves. This makes a brewing session more like 4 1/2 hours with cleanup, vs. maybe 1 hour with Mr. Beer, so many do both.
- All grain. Grains, hops, nothing else.
As to your question - when is it a must to do ____ or not to do ____ - beyond sanitizing things, NOTHING about brewing beer is a must. The worst screw-ups (both process and people making beer) usually result in drinkable beer. The reality is that we all have different tastes, and different abilities to taste. Someone like Josh picks up a beer, drinks it, and probably can say all sorts of things about it. You or I pick up the same beer and can't say have of those things, some because we don't know them, many because we don't have a palate that notes them. So you make what you like, and do it the way you want.
For me, the best learning is from comparison. Always make Mr. Beer refills, or recipes, EXACTLY AS STATED before you start changing things. Then, when you change things, try to limit the changes to as few variables as possible. Compare the differences in a BLIND TASTE TEST (better yet would be if someone else brewed them and you didn't know the differences). When you compare them, see what you taste/smell differently.
I have to laugh when I read the notes on some sites like Beer Advocate and such. "Notes of ____ and ____. Aroma of a fresh ____". Brewmaster for that beer is going "What? There are no notes of ____ in that brew!".