As Google points out in their announcement, waiting for an email to send can take a few seconds, and if you’re a heavy Gmail user, that’s a fair bit of time each day. Background Send dumps you back into your inbox while the process is completed out of sight.
Often when I’ve finished writing a message I’ll include an attachment and then hit Send — this queues up the message to be sent once the upload is complete. Waiting for an attachment to upload and go out with an otherwise complete message is the biggest hold up of all, and after running a few tests it seems that Background Send doesn’t work while the upload is still in progress.
Background Send is a great first step, and I’m all for saving seconds, but this feature will truly be worthwhile if Google can release an update that handles uploading in the background as well.