As mentioned previously the reason they are there is for safety.
In terms of street lights in residential areas they help the public feel safer walking about at night.
On the motorways they are generally there to improve visibility at night time, particularly around junctions where accidents are more likely to occur.
In the North West we have been experimenting with midnight – 5am switch off on the quiter bits of the network (eg M6 J27 – 31), and in some cases, where schemes are being carried out, a number of lighting columns now deemed to be not required are being removed.
However on busier sections and complex junctions lights are likely to stay on safety grounds.
Many of the existing columns on the motorway network are being upgraded to new, more energy efficient ones, however only a small amount are done a year due to the cost involved.