Imagine, that you have float field with 2 symbols after dot.
At the views page with filter you need to filter values by all range of module of user's float input.
Ex: user input 2.45, we filter all values X>=2 AND X<(2+1)
With the standard accessories of views you can filter with two filter “from” and “to”. But in FO we have only one field.
To solve this problem we can use hook_views_query_alter.
We already have a view. To make our hook work, we must set the unique tag to the view, to check it in out hook. mymodule_query in example code.