Earth seems to be pretty darn important to the Federation, beyond being even a founding member - even the alien president seemed rather invested in the paradise of Earth. I don't think it's vitally important to the Federation politically, though, asides from being the capital, and thus being where all the important politicians are.
However, Earth does seem to be the keystone of Starfleet. That would make it an important place for any attack, and why Starfleet is so keen to defend it (as opposed to, say, Betazed
). As Starfleet is the defensive force of the Federation, Earth is important to the Federation. Therefore, Layton would want to defend it, even to the point of martial law (and of course, he's the one most qualified to lead it, since he's the only one taking the threat seriously...).