It allows re-connection to an SSH session after you are dropped from what I understand.
This is for people who are on cell connections/spotty internet.
That was the idea, but it needs server support and the server side was never implemented in OpenSSH.
Perhaps roaming makes some kind of man-in-the-middle attack possible?
http://linux.die.net/man/5/ssh_config contains no mention of it, and DDG hits a reddit thread of 2014 asking the same thing (and giving an indication that it was also subject to another vuln) and they stated that it was added undocumented but "it does nothing yet"...
I found a commit message saying "Request roaming to be enabled if UseRoaming is true and the server supports it." So in addition, what is "request roaming"?