Every email you process takes time. Sifting through unnecessary emails becomes really frustrating. Don't clog up someone else's Inbox with unnecessary emails.
A good rule of thumb for whether an email is unnecessary is if your email comprises of less than 5 words (e.g. "OK" or "See you then") it's likely it doesn't need to be sent.
Naturally, an exception to this rule is when positive reinforcement should be used when someone has done a behaviour that you like and want to encourage. E.g. "Thanks for being proactive and pointing out that improvement."