sp_executesql -- one of the most important tool

Declare @killstatement  nvarchar(10) --- note sp-executesql only accepts nvarchar

 Begin 

  set @killstatement = 'KILL ' + cast(@session_id as varchar(3))

            exec sp_executesql @killstatement 

when some function expects uniqueidentifier keys but wont allow any parameter or variable 

for example kill 69 

you can not pass variable containing 69 

for example @mnop nvarchar(5)

@mnop = 69

kill @mnop -- will be wrong

or kill '@mnop' -- will be wrong 

the only way possible is above way using sp_executesql