Necessity of motor contactor with soft starters

you will need a motor starter before your soft start it not only starts and stops your motor it also gives you motor protection

I have not seen a soft start with over load protection

It depends upon the design of the soft start. If it is a soft starter, the word "starter" should imply that is has overload protection and a means of on-off control. In that case, an additional contactor may be unnecessary, but it might still be a good idea (see below). If it is a "soft start controller" however, then it could be a ramping device only; without necessarily providing the on-off control and/or the overload protection. In those cases you may need a motor rated contactor ahead of the soft start controller to provide the on-off control, as well as an overload relay somewhere in the circuit.

There are still other reasons for using a contactor ahead of a soft starter however. One is that it protects it from damage cause by voltage transient spikes such as those caused by grid switching, capacitor controls or lightning while the motor is not running. Most soft starters have at least some RC snubbers built-in, but these are not really intended to be transient protection. Some people use MOVs as well, but they have a tendency to fail catastrophically and even if they don't, you may not realize they are no longer functioning after being hit. Using a contactor as a line isolation device when the motor is off provides relatively cheap and reliable insurance.

Another reason why some people specify isolation contactors is that SCRs, the power devices in soft starters, allow a small amount of "leakage current" through them. The amount of current is very low, in the milliamp range, but the voltage is at whatever the line voltage is. With a digital multimeter you can read the line voltage on the load side of a soft starter that is in the Off condition, a fact that bothers many safety engineers (by the way, if you use an old analog D'Arsonval meter, the meter burden is enough to draw down the leakage current and it will not show voltage on the meter). By using a line isolation contactor, the air-gap of the contactor makes it so that no potential is measurable on the motor leads.

You may have to use a field Emergency Shutdown button, depending on the application and local regulations. In such a case a motor starter (contactor) is an imperative.

No need. SFU or MCB at starter I/P is good enough. If you require any emergency stop use MCCB insted of MCB and you can use Aux holding coil for remote Emergency stop button.

soft starter is not a Variable Drive which runs on continuously on the Semiconductor-Cicuitry. And it is not also Soft-Shut-Down.

It basically starts on ZERO_CROSSING & gradually reaches to full speed at a Set-Slope. After that CONTACTORS take-over to release the SoftStart Circuitry from taking load . Contactor is in Start-Stop Circuitry in which Emergency-Stop can be added where needed.

yes,it so good to use contoctor bcs to protect both motor&soft from power inturruptions.