Mr Barr has no illusions that he will win. He indicated that he can no longer be a part of what the Republican Party has become.
I think he is jumping in at this point because of what he described as "the perfect storm" scenario, with the Democrats polarized against each other, and the Republicans with a candidate that no one really wants who was picked from a bunch that not many really wanted. He wants to get people to start thinking about possibilities rather than sticking with the two party system.
I've been under the impression that if a 3rd party candidate receives 5%+ of the vote, the party then that party would be included in future debates, etc. If so, having the general public (the semi-interested, sound byte kind of folks) exposed to libertarian views might help reign in the other two parties.
I'm curious if Barr, with some name recognition (particularly among conservatives) might fair a little better than the usual no-name that the Libertarians run.