Interfaces have limited real estate. Interface options need to fit on the screen in order to be discoverable and accessible. This is different from the back-end. APIs have practically unlimited surface area.
Interfaces are also one-way, in the sense that you can't take features back. You can only get away with removing features in the very early stages of a product with extremely few customers. Once your customer base is larger, there will always be someone who found a way to use that feature in some unexpected way and now depends on it.
These facts make empty space in the product valuable. The value of your option to use the empty space in the future is high.
If you have an idea for a feature or a button, consider how likely you could be to have a better idea in the future about how to use that space. Always weigh this "option value" against the value of using the space now.