Your garage should be a space for you to store your vehicle and outside equipment. It should not turn into a place where you store everything that you can't fit in your home. You should get a storage unit for things that don't fit in your home and save your garage for your vehicle and outdoor stuff.
Item #1: Electronics
First, if you have any electronics, you will not want to store them in your garage. Items in your garage will get exposed to humidity, and your electronics can get damaged. If you have electronics, such as a nice television or a game console, you will want to put them inside of a nice and dry storage unit so you can use your electronics when you take them out of storage.
Item #2: Rugs
Second, rugs may take a lot of wear and tear in your home because they are being stored in a stable environment. When you put a rug in your garage out in the open, it can easily get damaged by pests, such as insects and mice. Additionally, the humidity in the garage could also allow mold and mildew to grow on the rug. Putting your rug in a climate-controlled self-storage unit will protect your rug from exposure to pests and will protect your carpet from exposure to humidity.
Item #3: Furniture
Most furniture is not going to do well in your garage. Wood-based furniture needs to be stored in an area where the temperature is consistent, so the wood doesn't get warped. Upholstery needs to be stored in areas that are not damp so they don't develop mold and odors. Even metal-based furniture does better in a storage unit, where moisture will not cause rust.
If you have furniture that you want to use again and keep in good condition, you will want to place that furniture in a storage unit. Furniture does better when it is stored in a stable environment.
Item #4: Books
It can be fun to build up a large library. However, taking care of that library can be a lot of work. That is why you will want to have somewhere to store your extra books while you rotate them in and out of your home library.
Books are sensitive to moisture, and when they are exposed to moisture, they can easily get damaged. That is why you are going to want to store your books inside of a secure storage unit.
If you want to keep things that don't fit in your house and keep your garage open for vehicles and outdoor equipment, you are going to want to rent a storage unit where you can store your electronics, furniture, books, and rugs and keep all of these items safe from damage.