Here is an older forum post about soft start

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BlueHeeler said:

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Can someone please explain how the soft start actually helps? Don't these things just ramp the voltage to lower the inrush current? What is this really accomplishing? I read something about this limiting the start up torque and keeps internal stuff from breaking over time? I call BS. My personal opinion is that it these things are snake oil.

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You're pretty much right on with ramping up voltage/lowering inrush. Motors heat up a lot during start-up. A soft start is basically a rudimentary VFD controller. For normal use, lower inrush means less heating during a motor start. Because heat is basically I^2*R, doubling current will quadruple the amount of heat produced in the motor coils during starting (simplifying).

Less heat on the motor coils will prolong their life, but the question is "how much?". Ben's opinion - direct-on-line motors are designed for normal inrush, so the cost of a soft-start for isn't usually offsetting enough wear/tear on the motor to make it worth the cost. Can't speak to the compressor itself.

Where soft-start really shines is for home generator set-ups. The single biggest limiting factor for a genset is almost always the starting demand from an HVAC compressor. You can buy a much smaller generator and pair it with a soft start and save a lot of money overall.