A groundsheet is critical for staying dry. If you don’t use one, and it rains even a bit, you will most likely wake up wet and profoundly unhappy. However, a solid tent plus a groundsheet can keep you dry in light rains or even moderate drizzles. Think of it this way: Groundsheet equals happiness.
Are tents okay in rain?
The most important thing to keep your tent dry in the rain is waterproofing it. While most tents are water-resistant, they probably aren’t waterproof.