Why was the army of the dead at Minas Tirith?

[Tuor in Gondolin]

Speaking of elves, here's a puzzle. PJ originally wanted Arwen leading the
elves to Rohan (where they were about as effective in
survival as the Crack Suicids Squad in Life of Brian):

Suicide Squad Leader: We are the Judean People's Front crack suicide squad! Suicide squad, attack!
[they all stab themselves]
Suicide Squad Leader: That showed 'em, huh?

So if she's there, and survives, then what! Oh, I know. She rides
with Merry and Gandalf to Minas Tirith on a really strong
Shadowfax.