Erik Darling shows how variable definition can be important, even without implicit conversion:
SQL Server makes many good and successful attempts at something called predicate pushdown, or predicate pushing. This is where certain filter conditions are applied directly to the data access operation. It can sometimes prevent reading all the rows in a table, depending on index structure and if you’re searching on an equality vs. a range, or something else.
What it’s really good for is limiting data movement. When rows are filtered at access time, you avoid needing to pass them all to a separate operator in order to reduce them to the rows you’re actually interested in. Fun for you! Be extra cautious of filter operators happening really late in your execution plans.
Click through for Erik’s demo.