Why?

In this way you can share from your host multiple ssh keys without copy/paste its on each VM. All ssh key are centralised and that's better when working with a lot of VM.

How to?

On the host side

In ~/.ssh/config:

#share ssh agentwith VM
Host *
 ForwardAgent yes 

On the guest side

In Vagrantfile:

config.ssh.forward_agent = true