[Iscool]

The obelisk is there because it stood in Nero's circus when the first Christian martyrs were killed. Period.

For what purpose did it stand there and why was it never torn down???

[Campion]

"For what purpose" ... I don't know, decoration I guess.

"Never torn down" ... It was "torn down," it was moved to the square in front of St. Peter's. The whole point, which certain people seem to be missing, was to stick the proverbial finger-in-the-eye to Nero and the rest of the persecuting Caesars ... as in, "Hey, turkey, WE WON!".