The motivation is using that syntax later for promoting the specified variable to a different scope like:

VAR    ${X}    scope=SUITE

This enhancement itself is proposed by #5369 and it is scheduled for RF 8.0. The syntax currently creates a variable with an empty string as its value and that needs to be deprecated before the syntax can be changed.

We need to decide should we deprecate all VAR usages without a value or only when it is used with a ${scalar} and an explicit scope is specified. For details see this comment of the aforementioned issue.