As a more fun or absurd anecdote, I reviewed a change in a SQL server stored procedure that merely copied the parameter value into a variable, solving a significant performance issue.

The query optimizer on first invocation determines the optimal query execution and resolution approach. For a stored procedure with parameters, parameter values can change what the best approach is.

In this case, the parameter could hold a record ID or null for all records. Because it included sub queries, whether it's one or the other makes a huge difference on the query execution approach.

Using an intermediate variable prevents it of taking and remembering the optimization path for the other case.