These numbers are usually used by companies who want to work in an area, but don't actually have premises there. For example, a small company might operate within a region, but cannot afford to have offices with staff in all of those areas within the region. Having a different phone number advertised in each area allows the company to appear to be more local than it actually is, thus more likely to attract trade than if the area code was for a town 30 miles away.

The number itself is pointed to an existing phone, so doesn't involve installing expensive equipment. It is just routed to an already active landline or mobile phone. You can usually change where the number is pointed to quite easily, which is helpful if you change business premises, are out on the road or away from your desk for a while. This means that you can just give out one phone number, but pick it up on different phones. HOWEVER, always check when searching for local number providers what their policy is when it comes to changing where your number is pointed.