Data might have super-human abilities but that does not make him a super-human poker player. On his first game, we see him thinking he got this all right and that he is sure about winning because he is a super-computing living machine.
He soon realizes that it takes a lot more that computing skills to play poker and he got beaten by Riker.
His visor is only an item that some poker players use to try and distract the adversary from looking at their eyes. Data has done a lot of research before playing poker. He then realized that some of the poker players were using things like that or glasses in a poker game, but he did not fully understand this concept until he got beaten by Riker and then decided to go further into his learning of poker.
Off course in real life these were originally use by people involved in accounting, auditing, economics, and budgeting. The phrase "green eye-shades" can be applied to individuals who are excessively concerned with financial matters or small and insignificant details (sounds like Data to me). -Source: Green eyeshade-
I just want to point out that Data was not suffering from eyestrain, wich gets me to conclude that Data is using this for style.