[williebfree]

The rule exists because some coaches send substitutes to the table in a "staggered" time manner. If officials were required to call the subs on the court prior to the first attempt they would often times be beckoning subs twice as much.

As mentioned by Jeremy Hohn, subs should be beckoned prior to the LAST FT, OR when replacement of a disqualified player is required (most often 5th foul DQ).