Before 1066 English spelling was quite simple, but during the next few centuries French, Latin and Greek words entered the language - and major pronunciation changes.
Over time, as pronunciation changed and new words entered the language, the match between letters and sounds broke down. As a result the spelling became incoherent.
Like all human systems, spelling occasionally needs modernizing. For ease of reading and writing, spelling should show pronunciation and pronunciation should determine spelling. If the match between letters and sounds breaks down, then learning to read and write becomes harder and all education can suffer. The lack of literacy in Britain is estimated to cost the nation £10 billion a year.*
For this reason, most languages have modernised their spelling in the 20th century. English, however, has not done so systematically over the past 600 years.
Although the British House of Commons actually passed a Spelling Reform Bill in 1953, nothing was done about it. Now it's up to the British people to decide if they want it enough to ask for that reform to happen.
*The report by Ernst & Young from which this figure is taken is quoted in a government paper 'National Strategy for Neighbourhood Renewal'.