Writing numbers in expanded form refers to writing them as units that are added together, rather than as a single numeral, such as 7,000 + 900 + 20 + 4, instead of 7,924. By writing numbers in expanded form, a student, often an elementary-level learner, can show that she understands what smaller numbers make up a larger number.
An expanded form number contains an entry for each digit in a numeral, so that, for a number in the billions, a student is showing how many billions are needed, as well as how many millions, thousands, hundreds and tens are needed.
Using expanded form helps students to learn about place values, so that they can easily gauge the value of a number without consciously having to think of what smaller units comprise it.
Learning to write numbers in their expanded form also helps students learn how to say numbers out loud. When a child can say that 9,876,543 is "nine million, eight hundred seventy-six thousand, five hundred and forty-three," she is demonstrating an understanding of how each place value operates with the others to create the larger number.
Practicing with expanded form numbers is a good exercise for English-language learners, who can use the exercises to familiarize themselves with English terms for numbers and their divisions, such as hundreds, thousands, millions, billions and so on.