In iOS 10 Messages get a huge upgrade and that carries over to the watchOS 3.0 update. With this installed on your Apple Watch you can now communicate in new ways.
One of the very interesting additions is the option to reply with handwriting to scribble a quick message when you cannot use dictation. Sketch out the letters and the Apple Watch turns it into text that you can send as a reply.
You'll also gain access to stickers, fireworks, invisible ink and all the other new iOS 10 Messaging options. It's also easier to send replies to people, send a heartbeat or send larger emoji.