I see the difference in number between the following:
बिल्ली जानवर होती है and बिल्लियां जानवर होती हैं
But why does it matter? When would I use one where the other would be improper?
They are interchangeable.
In English, we usually prefer the plural form 'Cats are animals' to make general statements but either the singular or plural form is acceptable in Hindi.