IF condition in Store proceure

HI Expert 

same structure availble in my Product database

my table:

CREATE TABLE A(ID int IDENTITY(1,1) NOT NULL,Name varchar(20) NULL)

THEN i am inserted records

INSERT INTO A VALUES ('Jan')
INSERT INTO A VALUES ('')
INSERT INTO A VALUES (NULL)

SELECT * FROM A

o/p:

IDName
1Jan
2
3NULL

Now my sp lijke below

ALTER  PROCEDURE AA
@i_ID INT
,@i_NAME varchar(20) =NULL

AS

 SET NOCOUNT ON  
  IF(@i_ID=0 OR @i_ID IS NULL)
   BEGIN
    SELECT @i_ID=MAX(@i_ID) FROM A WITH(NOLOCK)
WHERE (@i_NAME IS NULL OR @i_NAME=NAME )

END
ELSE
BEGIN 
SELECT * FROM A
END

My questiom:why here  IF condition written.this point i am not understand .

please any body can explain this part.this store procedure using application

 IF(@i_ID=0 OR @i_ID IS NULL)

   BEGIN
    SELECT @i_ID=MAX(@i_ID) FROM A WITH(NOLOCK)
WHERE (@i_NAME IS NULL OR @i_NAME=NAME )

END