I have a few emergency blankets tucked into my Bug Out Bag. They are used to retain heat. You can buy inexpensive blankets, or you can also invest in the larger, thicker ones, depending on the space you have. If you live in a hot climate, the inexpensive thin ones would be fine, but if you live in a climate that is fairly cold most of the year, you might want to invest in one of the better ones.
If you watch Discovery Channel’s Dual Survival, you can see how you can use an emergency blanket to make areally warm shelter, even if it is cold outside. You essentially make a traditional tarp tent, with one side facing the fire, and place the emergency blanket inside the far side tarp. As heat comes in the one side, it will hit the emergency blanket on the other side and reflect back in. On the Discovery Channel, their shelter was 70F inside while it was 30F or so outside with snow falling.
Definitely keep the blankets in their packaging until you actually need to use them. They are pretty thin and can tear easily, and trust me, you will never get them perfectly folded again!