(Contains spoilers from Fishman island arc. If you have not made it to the Fishman island arc or beyond that I advise you to not read unless you have already read the manga.)
I think that we should all have the same idea of what The Noah is to be used for. But I cant read minds so here’s what I think. But first a little recap on what The Noah is. The Noah is a huge ark that is introduced in the Fishman Island Arc after the two-year time-skip. The Noah was not to be moved from its resting spot until the appointed time it’d be put to use. Which in this case is right around the corner. So what is it’s use? My guess is that it’s a ship used for evacuation. In the Fishman island arc, Madam Shyarly made a prediction that Luffy would destroy Fishman island. Before you ask, yes, this theory is also related to the redline theory. Now The Noah’s approx. size is about half of Fishman Island, and the total population is about 5 million. If that’s the case I believe that The Noah was built to carry all of the islanders up to the surface safely. I say surface because the relocation of Fishman island was discussed during the reverie. If you don’t know what the reverie is, it is a council formed by the world government. It consists of 50 of the worlds leaders. The main reason Fishman Island was discussed to be relocated on land is because it was deceased Queen Otohimes dream. Anyway, The Noah has chains attached to it for the sea kings to pull.

This means that The Noah was made for the sea kings to pull it. Now about the redline theory. Since the redline is to be destroyed that means that Fishman island would be destroyed along with it since it’s right under the redline. Which also supports Madam Shyarlys prediction. But I believe after the Wano arc concludes Oda will start revealing answers to these questions more frequently.
But what do YOU think the Noah’s for?