Mailplane was created back in the days when IMAP support wasn't around in Gmail and you needed to access your Gmail account from your desktop.

Nowadays you can consult Gmail from any mail client like Mail.app or Thunderbird and there are even browser plugins that will do the checking for you. However, Mailplane is made specifically for the use of Gmail and in my mind, is the best way to manage multiple accounts at once.

Mailplane works outside of your browser to manage your Gmail accounts. Along the top of the interface, you have buttons for replying, sending, printing, forwarding etc much like a normal mail client. There are also buttons for quickly adding photos (with special integration with iPhoto) and viewing downloads. Alternatively, you can drag and drop attachments such as photos and documents onto mail while you're composing it which is a very nice feature of Mailplane. A similar feature also works for taking screenshots and sending them instantly via Gmail.

You can add as many accounts as you want using the Accounts button. When new mail arrives, a Mailplane icon in your Menu bar will indicate how many are waiting to be read and each account features a number next to it indicating the amount of mail in that particular inbox. I like the fact that by clicking on the Menu bar icon, a drop down menu shows you the subject heading of unread mail in each inbox.

The big advantage I've found though is that managing multiple inboxes with Mailplane is so much easier than doing it in your browser. You can't have more than one Gmail inbox open in a browser and while you can't in Mailplane either, switching between them is so much easier.

If you only use one Gmail account, then Mailplane isn't exactly essential. If you do however, it's the best way I've found of managing multiple inboxes.