The reasons May is doing this are numerous - but when it comes to Brexit, this election really can only strengthen her hand, by given her a clear mandate as Prime Minister. That puts her in a stronger negotiating position for the UK. It also means that she won't be fighting an election campaign in 2019 as Britain exits, so any short term turmoil that Brexit creates won't be as likely to negatively impact her campaign then and that will be a positive as far as she is concerned.
Forget statesmanship, Brexit etc...this is old-fashioned party politics. May knows this is a once-in-a-generation chance to secure a huge Tory majority without even needing to try very hard. She’s finally been persuaded - perhaps against her better judgement, certainly against her repeated assertions to the contrary - that it would be daft to let that opportunity pass.