Because it is more efficient to yell at Alexa "Turn off the bathroom lights" than stand up, walk to the bathroom and turn them off.
Or better yet, having all your lights and things turning on as you walk in the door. Or having Alexa wake you up by turning on your lights slowly.
Or baking something and telling Alexa to set a timer. No more having to type into Google "20 minute timer" then remembering to keep that tab open and your sound on.