There are articles about good professional way of doing communications. One of the "rules" I often see there say that you must provide good subject for your e-mail. This makes live a bit easier for your e-mail receiver (who, most probably should do something for you, right?).
I agree with this rule and with the argumentation.
But, doing so I discovered another thing: finding good subject for an e-mail helps you to understand what are you actually talking about and what feedback/action you expect from the other side.
Several times having troubles to find proper subject for an e-mail I discovered that I cannot myself cannot answer the question "what do you want by this mail?" - good reason to think about changing the mail before sending it.