Drives

Can't say for sure without seeing your system.

But sometimes, a soft starter is supplied as an emergency bypass for the motor in case the VFD is out of commission, such as for maintenance or service after a fault. They will specify a soft starter because the power grid and/or mechanical drive train cannot take the strain of starting across-the-line (DOL).

That's definitely an odd configuration. I have used inverter drives in the past AS a soft start, but I've never seen them used in conjunction. You might want to ask the manufacturer that designed it why they are using a soft start with an inverter.

I agree with JRaef - there are so many possibilities, without knowing the system we can't start to postulate why there are both.

Are both the VFD and the Soft Starter driving the same motor? If so, what switching mechanism is holding them in parallel? You may find that one motor only has to run at a fixed speed (so they use a soft starter), the other has to vary it's speed to suit the process. The soft starter is cheaper than a VFD, and (especially with a DOL bypass) will generate less heat when running at full power, and less noise. Less heat = more efficient = less running cost.