When organisms are photosynthesizing, pH will go up. That's because photosynthesis uses carbon dioxide and coverts that to oxygen. If photosynthesis isn't happening, the carbon dioxide builds up and forms carbonic acid in the water, and the acid drops the pH.
Since photosynthesis happens in the light, it is totally normal for pH to have a 24 hour cycle, where it goes up when the lights are on, and drops when the lights are off. Usually it will range between 7.6 and 8.4, depending on when you measure it. Running a fuge opposite the main tank lights or 24/7 ensures that photosynthesis continues in some part of the system when the display tank lights are off, and this is why having a fuge on an opposite light schedule helps stabilize pH -- something is continually taking up carbon dioxide, preventing the creation of carbonic acid.
It's better to stabilize pH at some level (even if not ideal) than to continually put in additives to try to raise it up to what you think is ideal, and then crash it over and over. I'm a firm believer in the idea that the more additives you put in your tank, the more difficult it makes it to manage all the parameters together.