What is the purpose of the handbooks wich u can get as loot?

Title says it all

If you are talking about the field manuals then I believe it is to lvl up your person beyond the cap example would be max lvl is 500 then you get a plus 4 field manual now the max cap is 504 or your max lvl is 900 and you get a plus 30 now it is 930 lvl cap. This is going off your experience lvl and not your weapon parts which is a unlimited cap