One more time an explanation by George Mastros on this problem:
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This happens because the 2nd version will not return rows where the data is null.

Declare @Temp Table(Data Int)


Insert Into @Temp Values(NULL)

Insert Into @Temp Values(1)

Insert Into @Temp Values(2)

 

Declare @Search Int

 

Set @Search = NULL

 

Select * From @Temp Where (@Search Is NULL Or Data = @Search)

 

Select * From @Temp Where Data = IsNull(@Search, Data)

The second query will return 2 rows instead of 3.