When creating components I mostly use UI elements with specific sizes (based on our internal UI guideline). When I add UI elements into a dialog for example, I always know that the dialog needs to grow by 64 pixels for example. So, instead of doing math in my head or using a calculator it would be really convenient if I could just add 64 pixels to what ever the current height of the dialog is.
If the current height of the dialog is 187 pixels and newly added UI element is 64 pixel high, I would just type (after clicking height field) “+64” and press enter — the new value would be 251 pixels. This would significantly simplify changing dimensions and positions!
A visual example with different numbers: