Some countries have been insisting on this for decades. I recall this the first time I travelled to Africa in 1965. Over the years there has been much discussion (including in PPRuNe) about the efficacy of this. Personally, I think it's pointless. Bugs can get into your suitcase whilst you're packing to leave and the cargo bins can carry all sorts. Spraying the cabin is, to my mind, a bureaucratic exercise. If I'm on a flight where they are going to do this, I cover my face, nose and mouth with a good cloth and keep it there for as long as I can after the CC have passed down the aisle.