When engaging students in reading--whether that be literature, nonfiction, textbooks, newspapers, etc.--it is essential to ensure that your students can read and comprehend those texts. The ability to decode a text (i.e., being able to technically 'read' the words in a text) is insufficient for understanding that text or for fluency. Readability measures provide educators with tools to help gauge the grade-level equivalent of a given text so that they can then match texts to students' respective reading levels. Ideally, the text an educator assigns a student to read will be within that student's "Zone of Proximal Development" (Vygotsky).
Essentially, the Zone of Proximal Development is the area in which a learner is most efficient in learning. When content (or texts) are too easy or simplistic for a learner, she/he will be bored and have less motivation to learn said materials. When content is too advanced for a learner, she/he is likely to become anxious, frustrated, and/or give up and again not learn said material.
In reading, this means that educators should know a given student's reading level, provide that student with texts that are slightly above than that level (to challenge the student), and provide scaffolding to assist the student in meeting the challenge. Readability measures are designed to assist educators in this endeavor.
There are a number of important caveats to the use of readability measures:
1) Readability measures do not and cannot account for the content of a text. Students' prior knowledge and maturity levels are essential to understanding a text
yet these cannot be easily measured (and are not measured by readability formulas).
2) Most readability formulas do not take syntax into account. Poems, original (and older) texts, and other materials that use an unusual syntax (based upon today's common usage) may have a high readability--they can be easily decoded--but are nonetheless difficult for the average reader.
3) Readability formulas are not designed as evaluative or diagnostic tools and should not be used to evaluate a student's reading level.
The Flesch-Kincaid Reading Ease measure is built into Microsoft Word (in tools)
Lexile measures are one of the most commonly used readability measures/tools. Lexile is a company that provides a reading diagnostic test to determine a student's reading level and that provides a Lexile rating for thousands of texts. Most major school districts use Lexile levels. Many booksellers also provide Lexile levels on their products.