Why should numbers under 10 be spelled out in writings?

Question

I am a Japanese teacher teaching English in Japan. I was teaching the general rules in writing essays, and I taught to my students, "numbers under 10 should be spelled out, such as '3 days' should be spelled out 'three days'". Then, a student asked me "Why? I think numerals are easier to see than letters". I could not give a logical explanation for this question, and I just had to tell the student it's just a rule to do so.

So the question I would like to ask is, why, logically speaking, should numbers under 10 be spelled out in writings? In addition, it would be best if you could introduce me how they teach the reasons for these rules in native schools.
Thank you.

Answer

It is a general APA and academic writing standard that asserts that people should use numerals to express numbers 10 and above, and use words to express numbers zero through nine, but there are some exceptions. 
You can tell them to view the  APA Publication Manual, Seventh Edition to understand more details about professional standards of academic writing.
The rule about spelling out numbers can be viewed in Section 6.32 of the APA Publication Manual, Seventh Edition.

Why should numbers under 10 be spelled out in writings?

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