Why would the cabin crew spray the cabin before landing?
The sprays are (usually) insecticides used to prevent the spread of invasive species and diseases. 


  The process of ‘disinsection’ is required under the International Health Regulations of the World Health Organization (WHO) on flights to and from certain destinations to prevent infectious and contagious diseases carried by insects and other volatile bodies. Rules established by the International Civil Aviation Organization (ICAO) require that disinsection does not injure or cause discomfort to passengers or crew. These rules permit the use of certain insecticides, which have the approval of and are recommended by the WHO based on their effectiveness and safety.


From the US dept. of Transportation