What it means
Readability formulas such as Flesch Reading Ease and Flesch Kincaid grade level score a piece of text on two mechanical things: how long the sentences are and how many syllables the words have. They were designed in the middle of the last century and they measure difficulty, not quality.
Why it matters to a small business
Marketing copy is read on a phone, one handed, in a queue. Long sentences and long words cost you readers who were perfectly interested, and the writer is the last person who can judge this, because they already know what the sentence says. A score gives you a second opinion that has no opinion.
The bit people get wrong
A good score does not mean good writing. You can hit a very easy reading level with prose that is bland and says nothing, and the formulas cannot tell the difference. Use them to catch a paragraph that has quietly become a legal notice, not as a target to optimise.