Do manifestos matter?
The answer is "Yes, but...".
Manifestos matter because they offer a rough guide to what a party will try to do in office. They matter because they are the best guide to a party's current thinking and of the promises that they want to be seen not to break.
If an idea is in a manifesto, it gives the idea power if the next government then clashes with the civil service, with the House of Lords, or with backbenchers. If it's not in there, those people feel less inhibited about resisting it.
But, being only a rough guide, manifestos don't tell you the things that parties don't want to reveal - for example, Tony Blair's plan to retire halfway through the last Parliament. They don't reveal the promises that parties then break, such as freezing the top rate of income tax. And they don't mention the things that those writing them did not or could not foresee - the mounting death toll in Afghanistan; MPs' expenses crisis; the global economic crisis.
So if you pay attention to what's in a manifesto, you're less likely to get lost on your journey through politics. But even armed with your political rough guide, there's no guarantee that you'll reach your desired destination.