First is to establish a crucial cushion against her own party's far right wing. Realistically, the EU could give a shit about the size of her majority; they hold the cards in their divorce negotiation regardless of what British voters say or think. Once again Tory party divisions dictate British national policy disastrously.
Second, a 2017 election means the next vote won't come in the immediate aftermath, but some three years after, May's inevitable failure to deliver a positive result in those Brexit divorce talks. Win or lose, she's in the clear, and she will anyway attempt to blame her failure on the'saboteurs', in chilling echo of Mrs T1's 'enemy within'. She made a feeble effort to distance herself from the rhetoric of the rabid Tory press, but even young Nick was able to see through that.
And while elections are always a risk, with Corbyn 'leading' Labour, she knows she can cripple them, and the rest of the opposition, for a generation, or at least a few electoral cycles, and that's just icing on the red white and blue British gateau.