Basic answer is all of the above. There can be several different reasons for pinching off flowers:
When transplanting a new plant (putting it into the ground or whatever is going to be its final growing place) especially a smaller seedling, you would pinch off all the existing flowers to help the plant focus on making good root system and growing (the plant) bigger.
For tomatoes and other fruits, some people pinch off some of the flowers after the plant is established, to make the remaining fruits grow bigger.
Often flowers, and/or the main growing tip is pinched out early on to make the plant bush out more, make more stems to produce more. This is often done for flowering plants like petunias, impatiens. Pinch the growing tips off chrysanthemums (they usually say Memorial Day and the fourth of July) to make it branch out more and make more flowers.
Deadheading flowers is pinching off the spent flowers to keep the plant producing more. If you let the plant proceed to making seed instead of flowers, it won't keep flowering. This also keeps it from going to seed, which can keep it from spreading itself around too freely in your garden.