Different plants have different temperatures in which they grow best. Too cold and the plant's enzymes can't get anything done. Too hot and the plant will have to expend too much water in order to stay cool. Temperature of soil can also affect the plant indirectly by affecting the level of moisture in the soil; warm soils will dry out more quickly. Because everything plants do in soils requires them to move water through them and draw water out of them, a plant's roots won't be able to do much if there isn't enough water.

This said, different plants are better or worse at the various trade-offs this entails; some can work at colder temperatures than others, some can extract enough water to get by in more arid environments, some can tolerate higher temperatures and not need to evaporate water off to keep cool, etc. Also, fungi like warm, wet conditions as well, and plants don't do well when their roots are being rotted away by fungi, so it cuts both ways (and yes, some plants are better at resisting fungal infection than others).

Soil has to be at the perfect temperature for that specific plant. Soil that is too low or too high in temperature can slow down cell extension. The uptake of nutrients, accumulation of carbohydrate, cell maturation, and enzymatic activity is all affect by poor soil temperature relative to the plant. Plants need nutrients and water from the soil, be able to effectively participate in photosynthesis, have active enzymes, and have functioning cells for it to grow. A lot of the plant's health and growth comes from the roots, which can be affected by the soil. If the temperature change is too severe, the plant may even become dormant or die.