???
An improved hand with one is an improved hand with the other. Any deal will need to apply to both so if she doesn't have to pander to the extremes (on both sides of any divide, the crazies are not all on one side) then she can be stronger and more confident.
Also, as the FT article says, it removes the threat of an impending election at the end of negotiations. It was looking likely we'd agree to a three year* transition deal where we'd be out of the EU and thus no longer able to vote but still applying rules like free movement etc - arguably the worst of all worlds. An election would have taken place during this transition. A June 2017 election means that we can agree to a three year transition until March 2022 and then have a June 2022 election with the transition over and Brexit complete.
* Talk of a three year maximum to any transition originated from the EU's side.