"Because, unlike linear power supply (which fluctuates with load and input), an SMPS gives you constant voltage even with varying environment (ofcourse within a frame of limits)."

Incorrect.
Both linear and switching power supplies aim to give a constant output voltage even with variations in input & load - the differences is in the way they achieve this aim.

As a sweeping generalisation, linear supplies are simpler, "cleaner" but less efficient [1]; switchers are more complicated, electrically noisy, but more efficient.

Switchers also give the possibility to step the voltage up.

[1] The main exceptions being at low power and/or low in-to-out voltage differential.