But does it "strengthen their mandate"? It seems like a political ploy to use leverage on a single issue for short-term gain. Perfectly okay, but does it really change any facts on the specific issues, and don't lots of folks see through the gamesmanship?
There's a few things behind the election:
1) May doesn't have a personal mandate since she wasn't leader of the party at the last election, and was also unopposed in being elected leader of her party (and therefore Prime Minister) after Cameron stepped down. You could argue that doesn't matter in a system where we elect a party not an individual, but there has been a lot of criticism and undermining of her (including from her own party) about her being unelected, and in the past this has undermined other PMs including Gordon Brown, who never won an election as leader
2) Some are also arguing that the Brexit referendum isn't legally binding and that the Conservatives don't have a mandate to take us out of the EU since it wasn't part of their manifesto at the last election (holding a referendum was). This is an issue in the House of Lords (our second chamber) who have to sign off on all legislation. There is a tradition/rule that they don't oppose any legislation which was in the manifesto of the governing party when they were elected. Winning a general election with an explicit pledge to take us out of the EU would therefore make it much easier to pass any legislation arising from the EU negotiations.
3) Purely politically, it's a great time. May's personal approval ratings are about as high as they ever get for a PM, and her party has a ~20 point lead in the polls which would translate into a much bigger majority than they currently have. The main opposition are in complete disarray with a leader who is widely thought to be unelectable and whose own MPs are largely against him. And having 5 years until the next election means there'd be a good couple of years to allow things to settle down post-Brexit before going to the polls again. As it stands, we'd be leaving the EU in March 2019 and then having a general election barely a year later when it's likely things will still be pretty chaotic.
The only risk is that a large minority (48%) were against Brexit, and if somebody manages to rally them all together in an anti-Brexit vote then that could lead to the election being a lot closer than expected. Right now it's difficult to see a party or candidate capable of uniting that vote, but who knows, things can change fast in politics.