Having separate phone numbers means that you can easily identify incoming calls, and answer appropriately. You can turn one off when you do not want to be disturbed by work or prioritise during other times. 

You can give out a work phone number to customers, investors, colleagues and keep your private, family lives unaffected by the pressures of business. 

A second phone number allows you to protect your privacy. Whilst so many people are working from home, there is even more of a need to maintain the structure of a work-life balance.