A business umbrella can help protect you above and beyond your underlying coverage in the event of an unexpected lawsuit. Not only does it provide blanket coverage (up to your policy limits) over your primary policies... it also broadens the perils.
Examples of an Umbrella claim:
Driver hits another vehicle that exceeds your commercial auto limits
Customer injured at your place of business
Fire that starts at your business and migrates to your neighbors business
If you have a 1 million dollar limit on your underlying policy and have a 1.5 million dollar claim, your umbrella would kick in and pay the extra $500,000 on a covered loss.