Questions about swarm setup and `local` machine install from doc

Hello,

I am running my system under Linux and in the documentation swarm install I can read that I need to create a local machine.
Q1: What is this machine used for please - there is no reference to it after in the doc?

Q2: Also what does the:
2 Load the local machine configuration into your shell
$ eval "$(docker-machine env local)"

is actually doing in details please ?

Thanks