Soft-Wifi is short for software based wifi hotspot. Using these datacards with "soft wifi", you can create a wifi hotspot so that your other devices like pc/phone/tablet can connect to the created wifi zone and access internet.
These data cards do not have hardware capability to create wifi hotspot themselves. So you have to insert it in your pc usb port like any other data card and then use the software that comes with it to create a wifi hotspot. So a PC is must to use these data cards. The software actually uses the wifi of your pc/laptop to create the hotspot and then bridges the internet connection between data card and wifi. Once the wifi hotspot is on, the wifi of your pc will be behaving like a router to provide internet access to other devices, so you will not be able to connect to other wifi networks unless you disable the soft wifi.
You can use the data card like a normal data card by pluggin it in a usb port. If you want wifi access for other devices, you will use the software to turn on the hotspot.
The other kind of data cards like the Huawei E8231 have hardware wifi hotspot capability. They do not need a pc to create wifi zone. The data cards with hardware wifi hotspot capability are slightly expensive, but a wifi zone can be created anywhere without a need for pc. You can plug them to a usb wall adapter or a powerbank and start using wifi. Some even come with built in battery.