Why do 1 pF capacitors exist?
The smallest capacitor I've used recently, in a filter in a 6 GHz receiver, was 0.5&nbsp;pF. There were some 2&nbsp;nH inductors there as well, and you could argue that those could be made with a few mm of track. However, both were smaller than the equivalent way of implementing them in copper.

Perhaps more importantly than the size, is that they were discrete components. When I wanted to change the capacitor from 0.4&nbsp;pF to 0.5&nbsp;pF, to retune the filter, I didn't need to respin the board; I just changed the bill of materials.