One of the biggest troubles of traditional phone systems is having to deal with multiple providers. Your office building might only receive services from one phone provider, while a different office on a separate location may have to contract a different provider due to zoning issues.
On top of that, you may have to deal with separate service providers for your mobile and data services, making for too many contact people to handle only one aspect of your business. With VoIP, all these solutions can be merged to one provider, cutting out the need for separate services.