what is the usage of dynamic sql in procedures?

hi ,
 

recently i changed my job profile from admin to developer.
 
first time i have seen only procedures in my project , there is no normal query in application ,
 
for every thing they written procedure that too they included dynamic sql in procedures.
 
can any body explain in layman method i.e what is the main usage of dynamic sql than normal sql.
 
i refereed some web site but i am not able to understood