Personally, I'd look very closely at any records this returns and verify that they are normal. A previous developer could have left some sort of backdoor or other nasty surprise in the code/database such could only be tripped with a very specific condition.
Alternatively, consider the context around this code. What is done with those records? Maybe there is a very specific bug elsewhere in the code or in a front end tied to this database. Sure, the right solution was to fix that other bug. But, that may not have been an option. So, this strange bit of code "solved" the problem and was then promptly forgotten.
The client table has around 100,000 rows each with a unique clientId, none of which are returned from the CAST / ABS / SIN
I think you are right and this is a 'fix' for something lost to time. I am going to talk to the original dev tomorrow to see if they remember what it was for