Due to the mancufacturing process of tyres, they always have a heavy spot in the casing, and when the tyre rotates at speed, these heavy spots will cause it to vibrtate. Balancing rectifies this problem, but if not assembled correctly or if it goes out balance the vibrations can cause serious issues with handling, particularly at speed and may also damage parts of the vehicle if not fixed.
A new tyre should always be balanced when fitted, and the price you pay at easywheels.co.uk always includes this. Over time though a tyre will wear down and the tyre may ease out of balance causing the tyre to wear down quicker. We'd usualy recommend getting your tyres re- balanced every 6-7 thousand miles or earlier if you begin to notice vibrations.