Freebase nicotine (i.e. pure nicotine) is produced by deprotonating the nicotine compounds as found in tobacco.
While greatly effective, freebase nicotine is alkaline, therefore it becomes harsh to the taste when present in higher concentrations.
Then nicotine is presented as a salt when used for vaping. We mostly use nicotine benzoate, closely followed by nicotine citrate.
Nicotine benzoate and nicotine citrate have much lower pH than freebase nicotine, thus limiting harshness and throat hit of the e-liquid.
Additionally, the above salts produce effective Vapour at lower temperature, and are absorbed (and eliminated) quicker in the body.
Regarding coils, atomiser and devices "designed for nic salts" - that is advertising fandango.
Nic salts can be effectively vaped at slightly lower (~20%)temperature than pure nicotine, but you achieve that with any variable power / variable temperature setup.
Only with a fixed power pod system you must use a coil "designed for nic salts", which will run at a slightly cooler temperature.