Best Practice for Passing and Working With Parameters Security/Performance
He seems confused. Using the same value from a local variable should be no different to using it from an input parameter. Is your colleague getting the wrong end of the stick with regard to using parametrised queries (sometimes called prepared statements) instead of full ad-hoc SQL?

For example:

resultSet = dbconnector.getRS('EXEC sp_foo_GET \''+inputVariable+'\');


could be an injection route and moving the value between variables inside the procedure will not affect it at all as the injected code will not be seen by the procedure. If inputVariable were to have the value xxx'; WAITFOR DELAY '00:10:00'-- the SQL sent to the server would be 

EXEC sp_foo_GET 'xxx'; WAITFOR DELAY '00:10:00'--'


Your procedure would run with he input parameter set to xxx and would not know of the extra code at all, it is run after the procedure call has completed.

With a prepared statement, assuming your DB access library properly supports them and isn't doing ad-hoc silliness under the hood, this would not happen:

resultSet = dbconnector.getRSWithParams('EXEC sp_foo_GET ?', [inputVariable]);


Your procedure would be called with the value xxx'; WAITFOR DELAY '00:10:00'-- rather than that extra code being seen as something to execute.

Of course the technique might be part of a larger pattern he wants you to follow the purpose of which will become clear as you do more advanced things yet which actually does nothing useful (other than getting you into the habit of following the pattern) at this stage, but I suspect he is simply misunderstanding and therefore adding unnecessary work. Could you ask him to explain in more detail? Perhaps he could provide you with an example call that he thinks the pattern will protect against?

After Comment Re. Performance

As per your comment, while I doubt there is any security reason for this pattern the new explanation that it could make a performance difference is more valid though you might find in most cases it makes no difference and in  some it reduces rather than increases performance so benchmarking rather than blindly using the pattern is recommended.

The issue comes from query plan caching and how input parameters &amp; literals are handled differently to local variables. I'll not go into it into massive detail here as we are straying away from the original question (search in the MS documentation and the web generally around the phrases "parameter sniffing", "query plan cache", and "OPTIMIZE FOR, then ask a new question if you need more clarity after reading a few things) but as a starter-for-ten: in the example you give above what you are doing with the local variable is effectively the same as declaring the procedure with OPTION (OPTIMIZE FOR (@parAcct UNKNOWN)). The affect of this, as with any optimisation hint, can vary a lot depending on the queries being run in the procedure and the balance of the data touched by them.

The difference between using input parameters, literals and local variables with respect to cached plans can be significant when debugging performance issues (it can mean the performance profile you see when debugging/testing is quite different to that seen in production if you are not aware of the issue). http://www.brentozar.com/archive/2014/06/tuning-stored-procedures-local-variables-problems/ covers the issue from this particular context.