SMT has many benefits over leaded components. SMT capacitors and resistors are easy to parallel together, and quick to solder and de-solder. There are no wire leads to add parasitic inductance which result in greatly predictability of component characteristics. S-parameters for active components depend less on the test jig and are therefore more useful in simulation and building. A significant number of modern components are only available in SMT form.
Leaded packages do no lend themselves to microwave use.
SMT parts fit neatly across the gaps on VERO board. Anyone who has made a PCB understands the frustration of trying to work on two sides at once. SMT simplifies this, because you draw, load and solder all on the same side. Components can be used on both sides of the PCB, or a solid ground plane can be used on one side with holes drilled only for ground connections. - Where a pre existing circuit needs modification. Forgot to add that series capacitor, diode or resistor.