What is the use of aromatic notes of bittering hop single-purpose?

I have used low AA hops such as Saaz 3-4% in a bittering role and high AA hops such as Amarillo 8-11% in a Aroma flavour role, both to very good effect. There are no hard and fast rules on this. One thing to remember is that the flavour and aroma profile of bittering hops will come through in the final brew, even if you boil for 60-90 min.

All modern hops can be used to for early addition/bittering role, you will just have to use 3 or 4 times as many as a 'bittering' hop.

Also, many bittering hops can be used in the Aroma or dry hopping role.

All hops contribute both aroma and bitterness in varying degrees. Grouping them into 4 hard categories is somewhat arbitrary. In general they are grouped as aroma vs bitterness, but if you wanted to you could simply use more "aroma hops" early in your boil and still end up more bitterness (and vice versa).

This chart shows the different components of a large variety of different hop types:

(Linked file for high resolution)

I found the chart on this blog post about hops but actually I originally saw it at the London Beer Lab, who do an awesome course on brewing (we were the audience for their prototype course, which was pretty cool).

Basically, above the line will be flavour and aroma components; below the line is bitterness.

Alpha acids control bitterness, and are not that volatile. For that reason, bittering hops are usually added near the beginning of the boil.

Aroma and flavour (which are kind of the same thing) are modulated by volatile chemicals. Actually the volatility is the reason they have such a strong effect on this: they evaporate and can reach receptors in the nose. Their volatility means at higher temperatures, they tend to disappear; flavour and aroma hops are therefore added near the end of the boil (we've used fifteen minutes for flavour and five for aroma).

Some hops a lot of alpha acid but not much aroma, and vice versa. If you use "aromatic" hops for bittering, a lot of the volatile chemicals will boil off and you arguably waste those; if you use high alpha acid hops for aroma, you might not get as strong an aroma, but you probably won't get too much extra alpha acid either because the section of the boil you use them for is comparatively short. That's about it.

However, as you can see from the chart, most hops have a decent amount of both aroma and alpha acids. Moreover, the aromatic compounds fall into several categories. So, for example, you might want a specific aromatic note in your beer which happens to be well-served by a "bittering" hop. In that case, go for it.

I'm going to make a google sheets version of this, by the way, so I can work out hop substitutions—there are six degrees of freedom and forty types of hops here, so some substitution must be possible.

"Bittering hops" are those with high alpha acids that contribute more IBUs than one with less alpha acids. But a bittering hop that is not boiled for long (or added post-boil) will not contribute to high IBUs or bitterness, just aroma and flavor.

You can use a so-called bittering hop for aroma and flavor and vice-versa. There may be debate on whether that results in excellent tasting beer, but it's doable, and the flavor and aroma profiles of these high alpha acid hops are important in those cases.