Once upon a time, some examiners observed anecdotal patterns of depressed scores in Arithmetic, Coding, Digit Span, (and some include Information), in the context of at least average performance on the remaining subtests, all associated with inattention and impulsivity (aka ADD/ADHD). If a particular examiner thought there was a question of ADHD, and they also believed the ACID profile was legitimate (newsflash: it's not), they might throw in the optional Arithmetic subtest to validate their behavioral observations of inattentiveness and impulsivity.
