When and why should you write numbers less than 10 in words?
Many style manuals will have specific requirements regarding when to express a number as numerals or in words.

APA Style

The APA version 6 style manual has an entire section starting from 4.31 on how to present numbers in text.

The manual does include a general rule that numbers 10 and above should be expressed as numerals, but there are a wide range of exceptions. 

Should be expressed as numerals


Any number in an abstract 
Any number immediately preceding a unit of measurement  (e.g. 3 kg)
Any number representing a statistical operation (e.g., 4 times as many people)
times, dates, ages, scores, and points, exact quantities of money, numerals as numerals
numbers that indicate a place in a series (e.g., page numbers, Table 1, Table 2, etc.)


There are also further exceptions for when numbers should be expressed in words


Numbers at the start of a sentence or heading
Common fractions
Where the usage is common


Why have the general rule to express numbers below 10 in words?

Compare the following:


I had 2 apples. 
I had two apples.


and 


I had one-hundred and thirty-two choices.
I had 132 choices


A few possible principles:


Numbers less than 10 are easy to read as words; Large numbers are difficult to read as words and easy to read as numerals.
In general, conventions make life easier for writer and reader.
There may be a slight disruption to the look of and reading of prose when interspersing numbers with words.
A few numbers, particularly "1" (which looks a bit like "I") might introduce slight ambiguities.