You don't necessarily need both, but they're often sold together as part of one package policy which is called either 'buildings and contents' or 'all-risks'. The decision depends on what type of risk profile you have in terms of the types/frequency/severity of risks that affect your day to day operations, so speak with an expert if this sounds confusing! You may find that contents insurance is all you need if your business doesn't have many valuable items on-site or if you only operate out of a home office.