Adaptations of boardgames allow people to play a game in different circumstances than originally envisioned by the designer of the game.
This includes adapting the boardgame to run on a computer or with computer assistence, run it on a mobile device or on some game console.
Other adaptations occur when the game is modified to suit a different number of players.
The actual pieces of the boardgame can be adapted as well. Ranging from making the board to play on more robust/stiff to complete redesign of the pieces.
New rules seems to be the most available adaptation. This happens accidently when people misread the rule book; start mixing different versions of the rules or come up with complete new ways of handling events in the game.