By doing water changes you remove excess nitrates and/or heavy metals that build up if you are only topping off evaporated water.
This is why aquatic plantings are so important. With the proper balance of plants to fish, Nitrates will not build-up nor will heavy metals.
They help rid your pond water of organics that build up and may not have been taken care of by your filter.
With adequate surface area (SSA) and a functioning Carbon cycle this should, at most, only be an occasional and passing issue, like after a spawn.
And the fresh water also replaces minerals that are necessary for your fish.
This has always been an interesting belief. Notwithstanding that the minerals and trace elements that are required for fish nutrition are contained in most commercial fish food, one would need to know not only which minerals and/or trace elements needed replenishing and the amount needed for each one, but also the levels of each the source (replacement) water contained. Otherwise it is no more than a 'crap shoot'.
It
is a proven fact that any abrupt change in an aquatic environment, such as water changes, will trigger acute stress in fish. Given, in most cases, the obvious limited value of water changes, this is stress that can be avoided
