Whilst contents insurance will protect against theft and damage to your personal belongings, buildings insurance will protect you against the loss and damage to the physical structure of your home.
For most mortgages, having suitable buildings insurance will be a condition of the mortgage. Many lenders will try to sell you their own policy, but often a much better deal can be found elsewhere. Whether or not you have a mortgage, it is essential that your buildings insurance adequately covers you for the full cost to rebuild your home, not just the value that you paid for it.