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A soft starter is rated on motor full load current, with an allowance for long ramp times because the thyristors dissipate heat throughout the ramp. Limiting starting current to 300% instead of the 650% of a direct-on-line start greatly reduces supply voltage dip and mechanical shock.
Soft starter
rating >= motor FLC x duty factor; starting current = FLC x current limit setting
Yes, as the square of the current limit. At 300% current the torque is roughly (3/6.5) squared of DOL torque, so high-breakaway loads may not start.
Whenever speed control is useful during running, or when very low starting torque with full current limiting is needed. A soft starter only manages the start.