Indoor plants have pores in their leaves just like you do and these get clogged with dust and oils from the house and the kitchen and makes it difficult for them to breath and photosynthesize.
Just like you need to wash your face and hands, plants also need to have a wash every now and then.
To clean the leaves, just use a clean sponge dipped in luke warm water to wipe the top and the bottom of the leaves until they are clean. They will look nice and shiny when you’ve finished.