In the Evans white offers a P to divert the black Bishop on c5 and if black accepts, white can play c3 and d4 gaining control of the center and open diagonals to play Ba3 or Qb3. This allows him to generate threats against f7 and prevents black from castling K-side. If Black declines then white gains space on the Q-side.
The gambit is named after Welsh sea captain William Davies Evans. Evans (January 27, 1790 – August 3, 1872) was a seafarer and inventor. He invented the tri-colored lighting on naval vessels designed to prevent collisions at night. For this invention he was awarded £1500 by the British government and a gold chronometer and £200 from the Tsar of Russia.
Evans was most likely educated at Haverfordwest Grammar School. About the beginning of the century the family moved to Castle Pill, the name of an inlet of Milford Haven in Wales. He went to sea in 1804 at the age of 14 and served in the navy until the Napoleonic wars ended in 1815. He learned to play chess sometime around 1818 and was transferred to the postal department until 1819 where he served as captain of a mail ship named the Auckland which sailed between Milford Haven, Wales and Waterford, Ireland. During this period he played a lot of chess with a well known player of the day, Lt. Harry Wilson.
As a chess player, Wilson was one of the last surviving veterans of a group of players between what has been called the Transition School which was a group of players between Philidor and de la Bourdonnais. Beside his chess career, Wilson, who was described as a man who never made and enemy and never lost a friend, served as an officer in the Royal Navy. He died in Spring Vale, Isle of Wight in 1851.
It was some time around 1824 Evans invented his gambit and in 1826 he created a sensation in the chess world by introducing his opening in a famous game in London when he defeated Alexander McDonnell, the strongest player that Ireland ever produced.
In January 1840 Evans retired on a pension and spent his time at London chess clubs and traveling abroad. He died on 3 August 1872 at 29, Rue Christine, Ostend, Belgium and is buried in the old cemetery in the town. The inscription on his gravestone reads: To the sacred memory of William Davies Evans, formerly Commander in the Post Office and Oriental Steam Services; Superintendent in the Royal Mail Steam Company, and inventor of the system of tri-coloured light for shipping. Also well known in the chess world as the author of the Evans’ Gambit.
In 1832 the first analysis of the gambit was published in the Second Series of Progressive Lessons by William Lewis and the gambit became very popular shortly after that, being employed a number of times in the series of games between McDonnell and Louis de la Bourdonnais in 1834. Players such as Adolf Anderssen, Paul Morphy and Mikhail Chigorin subsequently took it up. Eventually however, Emanuel Lasker dealt a heavy blow to the opening with a modern defensive idea: returning the pawn under favorable circumstances. See GM Bryon Smith's article at Chessdotcom, The Evans Gambit: Modern Play
As a result of Lasker's innovation the opening was out of favor for much of the 20th century, although John Nunn and Jan Timman played some games with it in the late 1970s and early 1980s, and in the 1990s Garry Kasparov used it in a few of his games which prompted a brief revival of interest in it.
In the following game Evans wins brilliantly against McDonnell, but analysis with Stockfish shows that McDonnell missed a chance to save the game at move 16. Have engines finding flaws in the play of the great players of yesteryear resulted in a loss of respect for their play? Engines have made us all armchair Grandmasters, but they haven't helped us understand chess any better. All they have done is proved the old masters weren't perfect.