Now you're going to get people who laugh and say you don't need permission, but I was taught from a young age to ask before taking a leaf, branch or flower. Of course at that age, I was told to "listen" for it to say it was alright. I still sort of "listen" but it's an intuitive understanding not actually waiting for a verbal response.
Really it is just a form of respect. It's like saying "excuse me", "pardon me" etc.
You're approaching the plant as a living being and giving it honor and respect by not just ripping a piece of it off. If you really want to be formal about it, it is also "proper" to leave an offering at the base of the plant as an exchange.