I can't explain it in any very satisfactory way.

RYA Rule 37, in 1923 said

 RULE 37.- -ANCHORING DURING RACE.
A yacht may anchor during a race, but must weigh and recover her anchor again, and not slip.  No yacht shall during a race make fast to any buoy, stage, pier, or other object, or send an anchor out in a boat, except for the purpose of Rule 36 [Running Ashore, Getting Clear],

I can only speculate:

Along with John's comment above, a boat in a slip, on the beach, on a buoy, etc. might receive, for example, a different sail due to wind change, or have repairs done by someone not of crew via items not on board.

Where i sail, we can have adverse tide, and difficulty getting to or staying by the start line. We corrently kedge but it is not simple. If it were not for this rule i would have a mooring on the line to give me an advantage.  The same with a dinghy, you hog the beach till conditions suit to sail. The rule levels the playing field. If we could we would pay to have the appropriate mooring for an advantage. 

When the starting line was positioned close to shore and an adverse tide was running, I would stand in the water and hold the boat just behind the line. If the line extended to a dock, I would have held the boat from the dock instead. However, Rule 45 permits standing in the water but prohibits standing on a dock.

In very long offshore races, it is common practice to start races in the prevailing wind, whatever it is.  Similar to Mike's comment, in strong favourable tide with light winds, I've certainly used that rule to sit on a mooring buoy my estimate of 4 mins uptide from the start line and dropped the mooring just before the prep to allow the tide to take me to the line.  Without the requirement to be off the mooring at the prep, I'd have chosen a buoy nearer the line to allow the perfect start without risk of being OCS (I've never persuaded a crew member to stand on the bottom and hold the bow in 25 m water!).  

Thank you all for your insights!