Because the peat breaks down over time and becomes bad for the plants. How often you change it out depends on the size and depth of the bog, what you have mixed in with the peat, the water you use and how wet the bog stays, climate, etc., too many variables to be able to give a timeline for when to change it out. If it has been a couple years since the soil was new, dig some of the peat up from the bottom and smell it. If it smells bad, time to change it out. One of my bogs (in-ground) is roughly 5' x 3' with a depth of roughly 18" and goes about 4-5 years before the peat starts breaking down, this is in the Pacific Northwest, where we get lots of rain, relatively mild summers, and winter freezes. It is filled with mostly peat with some perlite and a little bit of sand, and I do not use any kind of mulch on top in winter, which may also affect how fast the media breaks down. Hope this helps.