Why do we need to know the height and weight of our Pokemon?

So far they dont, but may be in future updates they might matter.

It's entirely flair at this point, unfortunately.

Those stats have always been in the trading card game also...no purpose as of now.

I can see smaller Pokemon maybe being given a smaller cooldown period for their attacks to reflect speed and maybe bigger pokemon having a slight stamina modifier, but at the moment there's no advantages.