You vote for who will represent your constituency.
It's a mixture of whether you think a candidate will represent you well, and how much you agree with their party's policies and leadership.
An example. My mate is not a natural Labour supporter, but will vote Labour this time because:
a) His standing Labour MP, Dan Norris, actually turns up to debates, often speaks, and tries to elevate local issues to national level.
b) The only viable alternative is the Conservative, Jacob Rees-Mogg, who's been parachuted in after a boundary change. Google him – his opinions are laughable!
I'm in a LibDem/Conservative marginal and will be voting to boot out the sitting Conservative, mainly because of his dodgy expenses. But also because I'm a intellectual bleeding heart Liberal in my bones and want the LibDems to a have a fair and proportionate voice in parliament. A hung parliament ticks my boxes.