One possibility is that it is acting as a stabilizer of the enzyme. Sugars are often used for this purpose.

Hi Artem, as Adam said it is probably there to stabilize protein structures by keeping the solution at normal osmolarity. Plus mannitol is not easily metabolized so it will help keep contamination in check. 

Got it! Thanks a lot! Osmolarity sounds logical.

Dear Sir ..Its importance of the stability of the enzyme