When to use Nair?

[Zanestat]

I have started to play Rosalina as of a couple of weeks ago since she is no longer the flavor of the month, and I have been seeing a lot of top tier players use Nair, while for me I would rather use any other type of aerial. This easily means I am seeing it the wrong way, so when would I use Nair?
Thanks

 

[Parcheesy]

It's generally used as an approach by using it well before you plan to hit your target, then poking them with the last remaining hitbox on it. It functions as a decent anti-air as well for the times where they're diagonally above you and out of range of up-air. Finally, Luma's neutral air is amazing, especially when you autocancel it. I might be forgetting a few things, but it's an overall good move.

 

[Zonderion]

What Parcheesy said. I am in the same boat as you. It seems slow and awkward when trying to approach with an attack that begins behind you, but if you play with Luma next to you, he will do his nair which hits early and strong, if they shield or spot/air dodge Luma, they will get hit with the ending hitboxes of nair. Rosalina has almost no landing lag from her nair (if I'm not mistaken) which can allow for great followups, such as a boost grab if they did shield.

It is a little weird at first, but you will get the hang of it.

 

[iiGGYxD]

With the approaching nair, if you space properly while it's being blocked, you can throw out dtilt after as it comes out very fast. This catches people off guard all the time when they try to punish the nair on shield.

 

[Garde Noir]

Nair also has a more consistent hitbox than Fair from what I've found. It's more predictable for you to where you want to hit the opponent. Bair is even better because of its speed, but it's harder to get the opponent where you want them

 

[ParanoidDrone]

Nair stays out for so long and Rosalina has good enough air control that you can be behind someone, SH nair, and land in front of them in time for the final hitbox to land. Not that this will ever go smoothly against a person, but it's possible to sneak in a hit this way just because it lasts for-freaking-ever. The reduced knockback also makes followups possible where previously there weren't a whole lot.

It's also her safest aerial, I believe, since it has the shortest landing lag out of all her aerials and it has an autocancel window that you should land without issue if you SH nair without fast falling. Following up with a tilt or grab on shield or a dash attack on hit is perfectly viable.