VoIP is a technology which allows you to send voice over a computer network, including the internet, in a similar way to sending data over a network such as files or emails. The signals are called voice packets, and can be interpreted by all of the national and international telephone networks so that you can talk to anyone who is on a regular telephone number whether it be landline or mobile, and they will not be aware that you are using VoIP.
The simple answer to this question is because VoIP is an immensely flexible technology and it's use can bring significant cost savings. Telephone systems that support VoIP, such as Swyx and CISCO UC500, allow users to connect to the system over a data connection, either internally on the LAN, or externally from a broadband-type connection. The use of VoIP on the internal LAN allows users to take full advantage of integration between their phone and the applications on their PC such as Microsoft Outlook and CRM programs, in fact many VoIP enabled PBXs offer a soft phone option so that users can have their phone on the PC or Laptop screen. Both Swyx and CISCO UC500 telephone systems that Ring24 provide use VoIP technology to enable users to log onto the PBX from a remote location and work as though they are sat in their office.
Connecting voice over a data network enables businesses working over multiple sites to connect from each site to the same central PBX and route internal voice calls over the data network, rendering internal calls free of charge. Internet Telephony Service Providers offer SIP trunks which can be connected to a PBX supporting VoIP, such as the Swyx or CISCO UC500 system, and this gives a business access to flexible, cost effective voice access to the outside world via the internet.