Why should you sometimes truncate a word by leaving out a letter or group of letters (e. g., creat rather than create)?

truncation is not useful in most searches and shouldn't be used.
truncation will limit the results to only those you want.
use truncation when you aren't certain of the spelling of a word.
use it to broaden a search to include different forms of the truncated word.

The reason why you should sometimes truncate a word by leaving out a letter or group of letters (e.g., creat rather than create) is because you use it to broaden a search to include different forms of the truncated word.
So, for example, you may not be looking for create only, but for creation, created, etc. which is why you can sometimes shorten the word to take those derivatives into consideration.