I believe the intended purpose is to reduce the overall acidity of your body, which it will do (negligibly, maybe even immeasuably). Your stomach acid will compensate regardless, but, in doing so, it uses acidic compounds in the process to do so. Whether that is even beneficial in general is debatable at best, though likely not. But mixing in other acids does negate at least some of the alkalinity, which would defeat the entire point, if there is any effect from it.
Edit: Clarified my position a bit. I'm not suggesting that alkaline water is effective at doing anything at all, nor even that its intended purpose would be a health benefit.