It is done by all political parties so that they can work out which of their supporters have voted.
But how can they possibly know who their supporters are until they've voted? And even then, it's a secret ballot, unless folk are particularly keen to tell a stranger who the recipient of their vote was.
Surely, if they're self-identified supporters - members of their own team etc. - they will make certain that they vote. Even people who are (currently) members of one party or another may well decide to change their vote, come the election.
Or are they doing the arrogant politician thing of assuming that anybody who decided to honour their party with their vote at the last election is automatically 'their voter' thenceforth, whether they've done anything to win their trust again or not?
There also seems to be further arrogance in them 'reminding' people to vote in the first place. Everybody knows that there's an election on, so if they haven't voted, it doesn't mean that they have forgotten to do so; rather that they have decided not to vote because they can't be bothered, don't see any point in it, don't have a preference, can't hold their nose enough for any of the candidates etc.
Using your vote is exercising your choice; just as choosing not to use it - whatever your reasoning - is also exercising your choice. Personally, I think that looking who hasn't yet voted, going over and knocking on their doors and then 'encouraging' or 'reminding' them to come out and vote (whatever format that may take) is potentially stalkerish, coercive and/or undemocratic.
If you're obviously the best candidate, a majority will vote for you and you will discover that you've won once it's announced... and if you aren't the best candidate, why on earth do you think you should win in any case?!