If you are starting with seedlings or growing your plants from seed. They have likely been inside for a bit, and you need to harden off your plants before they are outside full-time. Plants can really struggle with sudden changes, and if they are inside your warm home and then outside all day and night, they may not like the sudden temperature change. Especially if the nights are still on the cooler side. You might think it will be fine to skip this step, but it can make the difference between a big, thriving plant and a smaller, stunted plant.