Soil temperatures are extremely important for seed germination. For example spinach seeds at 40 degrees take 23 days to germinate and only 12 days at 50 degrees. Green beans at 59 degrees take 16 days to germinate and only 8 days at 77 degrees. I have had green bean seeds rot in the ground because I tried to plant them when the soil was too cool. Tomato seeds take 43 days at 50 degrees and 8 days at 68 degrees. We could use a thermometer to measure the soil temperature but there's another way to know when to plant.
Or another way to help decide when to plant is phenology, the relationship that climate has between plants and animals. I made this handy chart to give us guidance.