True, She is unlikely to do so. But by having that power, She can (and in some countries in the recent past this has happened) can cause parliament to reconsider a controversial law, and in extreme cases prevent laws from being passed that would be harmful to the democracy (say parliament passes a law that dissolves democracy, gives themselves total power over the country with no say of the people, something that's effectively the case already but not written down of course, the Queen would almost certainly veto that law and dissolve parliament, maybe have the entire bunch of lunatics finally arrested and tried for high treason like they should have been years ago).
That's her main authority, keeping parliament/cabinet from becoming a real rather than a de-factor dictatorship.