I recently discovered you can bookmark you GMail messages. You may ask why you'd ever need to do this. Suppose you are looking for a specific email which is buried in your GMail inbox (which maybe 45% full!). When you find and open the message, the URL in your browser URL bar is unique to the email, and it can be saved as a bookmark. Later when you want to open the same message again, just open the bookmark you saved.
I think I'll find this useful when I get receipts for things I buy on the web. The receipts usually have a link to tracking packages or order status. I can bookmark the email making it easier for me to check later -- Beats having to do a search again.