I get a lot of mail each day, and often a lot of the mail requires me to do some sort of work to reply.
Often, I’ll open a mail and then realize that it’s going to be a while before I’ll have a chance to provide a good reply. But once I open it, the mail gets marked as read; so if I closed it, I’d probably forget about it forever.
I’d also tried flagging messages in outlook, but that had a few problems. I like to sort my view by conversation topic, which makes it difficult to find flagged messages. And flagged messages are still easy to ignore.

Thus I’ve gotten in the habit of just leaving the mail open as a way of tracking outstanding items.  It’s tough to ignore the message when it’s sitting on your desktop. Then about every two days, I’d plow through all the open messages and deal with them.

It seemed like a good idea … until my computer crashes or reboots (usually from an auto-install of a critical update).