Shells and limestone will slowly dissolve in the water, leaching out calcium (GH) and carbonate (KH). If you want calcium and carbonate leaching into the water, then this is a good thing. If you don't want these leaching into the water, then it's a not so good thing.
The parameters of the water will change with shells and limestone in the water. The change in water will increase with time, so if you neglect to conduct a water change in a reasonable time period, then the water in the tank can become significantly different to the water you use for water change, which may be an issue when you do get round to doing that water change.