The noun doesn't necessarily follow immediately afterward. "Ese" refers to "propósito". Este es el día. Esa es mi mochila. "Eso" is used when there isn't a specific noun to tie it to. ¡Ay no! Esto no es bueno... (Oh no! This isn't good...) Sí, eso es lo que dije. (Yes, that's what I said.) Sometimes it seems like there's a bit of grey area (like this sentence) where Duolingo will accept either the neutral or the gendered word.