Upholstery buttons have been used for centuries as part of the decoration of furniture and are produced in many shapes and sizes. Though most of them are covered with a soft fabric, there are hard ones as well, making your furniture potentially uncomfortable.
The truth is, in many cases modern upholstery buttons have no purpose other than decoration, and they can be removed without causing damage to your furniture if they become uncomfortable. However, they sometimes serve a purpose in keeping your furniture's shape.
Buttons are often used to cover up nails that have been placed through the fabric of your furniture to give your upholstery different shapes, like the classic diamond. The buttons are generally softer and more comfortable than the nails and it gives the upholstery a softer look. Buttons can also be used to hold thread that has been pulled through the upholstery. Cutting that thread could result in your furniture losing its shape or unraveling.
If you want to remove your buttons, examine them closely first to make sure that they are not an essential part of furniture. Don't assume they are merely decorative. If the buttons are loose and can be moved around easily without the shape of the upholstery changing, you should be able to remove them safely.