There are several posts here about the right ways and wrong ways to change water in your fish tank, and all the ways it can go wrong. Why do you need to change it? It would seem unlikely that water would wear-out, you need to manage the pH and actively filter to keep it clean, if you are managing it effectively why would you need to change it?
One of the primary reasons for water changes is to remove harmful waste and chemical compounds from your tank that tend to build up over time. This is especially important in new aquariums where there aren't bacteria and an ecosystem in place to handle any of these compounds organically. There are two compounds in particular that most people target for reduction by doing water changes: nitrates and phosphates. In established aquariums, these two things build up naturally and will cause algae blooms, stunted growth and stress your fish. In newer aquariums that haven't fully cycled, water changes may be used to reduce more harmful compounds like ammonia and nitrites if they get dangerously high and there are live animals in the tank.
Another important reason for water changes is that it can replace trace elements and minerals that are depleted from an aquarium's water over time. The fresh water or fresh salt water added during a water change will replenish some of these elements and minerals. Without water changes, an aquarium will require much more monitoring and dosing of supplements to maintain a balance.
[...] you need to manage the pH and actively filter to keep it clean, if you are managing it effectively why would you need to change it?
It's true that pH and cleanliness is important, but it's not necessarily the most important. You can easily keep a tank that looks clean and maintains a stable pH but the water is toxic to your fish or other critters. A lot of other levels need to be monitored that could be more crucial for your fish/invertebrates/corals:
- Calcium, magnesium, etc.