Serious question. I've been looking at docker today and I can't seem to grasp why would I use docker for dev and deployment over something like ansible also bot

Serious question. I've been looking at docker today and I can't seem to grasp why would I use docker for dev and deployment over something like ansible also both for dev (on VM) and deployment. I must be missing something. I now understand vagrant somewhat that it alleviates manual hurdle of VM starting and running, but not enough for me to use it.. yet I don't see where and how docker fits in. Would I be wrong if I thought of it more as a virtualenv(wrapper) but not only for python?