[Jprogramming] Use of (@) 'Atop'

[Martin Kreuzer]

Hi all -

Approximating Pi using the Monte Carlo method (circle enscribed in 
unit square) came up with

    pia=. 13 : '(4 * y %~ +/ 1 >: +/ *: <: +: ? (2,y) $ 0)'("0)
    (pia 10^(4+i.3)) ,:(4+i.3)
3.1524 3.1438 3.14106
      4      5       6

I then compared my version with the J verb at Rosetta Code and the 
only difference I found was the use of 'Atop':

piMC=: monad define "0
    4* y%~ +/ 1>: %: +/ *: <: +: (2,y) ?@$ 0
)

Q:
What is the advantage (in this particular case) of using 'Atop'..?
    ? (2,y) $ 0
vs
    (2,y) ?@$ 0

Thanks
-M