This document describes rules for writing BDD instructions. It is used as a guideline for proper stated and formatted instructions. Natural language instructions used for BDD are not absolutely natural language instructions. They're actually "naturalized" form of some specific command with ability to variate some part. So, from the technical side of BDD the strict rules are still necessary to avoid different forms of absolutely the same phrases but with small differences. It's not essential for understanding but it is critical for technical implementation.
This document contains examples of correct and incorrect phrases. If some phrase is described as incorrect it only means that it violates the standards though it can be properly built from the language point of view. These standards do not teach English. They just describe which subset of possible phrases is taken as acceptable.
And again, standards are designed for uniformity only. There's no strict dogmatum in them. If some phrases seem to be more appropriate but they violate the rules it's OK to adjust rules to current needs rather than restrict needs with existing rules. The main thing is that there should be some uniform rule set which is kept within the project. Current document can be a basis for such standards