Have you ever caramelized onions on the stovetop? First you melt butter in a big heavy pan, then you add a bunch of onions, and stand there, stirring and cooking, for an hour or more. Stand there. Stir. Monitor the heat. Check for any signs of burning. Stand there. Stir more.
Now how about this: stick butter, onions, and herbs in your slow cooker. Cook for a couple of hours, stir once, then cook more for hours and hours, blissfully unattended with no worries about burning. This is a better way!