Why is a pressure canner needed when canning?
Physics stops you from heating up liquids that consist of mostly water to temperatures above (roughly) 100 C. 

The temperature of your heating element can be set higher, but neither the temperature of the water bath nor the liquid in your jars can go higher than the boiling point where water changes from liquid to vapor  - which is 100 C at normal pressure 1. To raise the temperature, you have to increase the pressure, so that the new boiling point is at or above the temperature required for safe canning. A pressure cooker may be suitable for the purpose, provided the model is designed for the specific pressure and allows setting/reading the specific pressure or alternatively, temperature. Pressure canners on the other hand are specifically designed for that purpose and marketed as such.

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1 Note that 100 C is correct only at sea level and given a few more constraints. At higher altitudes the boiling point may be significantly lower. For the answer here, 100 C is close enough. (Or far enough from safe canning temperatures for non-acidic items.)