You can have the best "sizzle" for a product ever written; something that will produce the urge to buy in 90% of the listeners... BUT... if you promote a $50 item to a group of people for whom $10/month is their average wage it doesn't matter HOW good the sizzle is, the fact is that they won't buy it (or the numbers buying it will be much smaller) and you've spent your wonderful words on a market where your efforts are going to be minimized.
Compare that to addressing the same Wonderful Words to a group of people with a six figure income. You get more return for your effort.
To me, that's the important part of marketing. I don't have all the time in the world and so I carefully target my products (comic books, as an example) to the group most likely to buy them. They, in turn, often spread the word to friends by passing the books along. If I include email and URL with them, a viral marketing effect takes place -- still among the target market (people who like comic books.)
Doing a "sizzle ad" to a group that knew me would get some results, even if what they know me for has nothing to do with the comic book (the curiosity factor.) But if I were to do the best ad for comic books (these are small press comic books, by me, nobody famous involved, not collectors items) to a group of nursing home residents, my returns would be thinner.
...and that's my reasoning. Maximum return for your limited marketing time. Yes, all markets could be touched with the right ad -- but because I have a very limited time, I pick and choose my targets.
The Five Minute Mentor