Tarps are an enormously useful item for creating or roofing a shelter. Alone they work great and with with few accessories even better.
Not only does a tarp protect against rain and snow, it also gets you out of the sun on a hot day, out of the wind on a windy day, and acts as a barrier to frost and condensation. I’ve learned from experience about the latter (which is also why I often have an MSS bivy with, but that’s another story).
It can be tempting to skip the tarp (or tent) and sleep in just a bag with a clear view of the stars above you. What you might not expect is waking up with your bag damp to soaked from condensation. As temperatures drop and humidity drops out of the air, the bulk of it will condense on the final layer of your set up. This is where that extra layer between you and the air around you comes into play, it gets wet and your sleeping bag remains dry (relatively). This isn’t always an issue but it’s a very annoying one when it is.