Knowing the temperature of your soil is especially important when direct sowing certain crops outside, for example beans, cucumbers and some cabbages specify a minimum temperature in the soil before sowing. They can't sprout at a lower temperature than this, and risk rotting in the soil before it warms up.
The temperature of the air is not a reliable indicator of the temperature in the soil, a soil thermometer will tell you for sure before sowing your seeds.
