Another way to understand this is that some functions are on another context rather than the place where you are writting the expression.
This is true basically for all functions that have a "Row filter condition" in it since those move you to a particular table context so column names inside those expressions/functions refer to that table, not the one you came from. But, in order to use the columns from the table you came from and bypass this "limitation" were AppSheet expect you to write column names from the function's table, you can use [_THISROW] to force it to consider the current row's table's columns.
Then you have nested functions with this kind of behaviour and [_THISROW-N] is also a thing, and you can see an awesome explanation of that one here:
Back-references within nested queries ([_THISROW-n])
LinkedIn
Twitter
