Over the lifetime of your piano, a huge amount of dust will accumulate inside the instrument, which can act as an abrasive on moving parts. It can dull hammer action and cause noise. This must be cleaned by a professional piano technician, but can be avoided slightly by regularly dusting the cabinet with a soft dry cloth or feather duster.
The keyboard should also be wiped periodically with a soft dry cloth. If the keys are dirty, wipe them with a solution of vinegar or lemon juice and water on a cloth and wrung out well. A good habit to adopt is never play a piano with dirty hands!