What exactly is Kuva used for?

[★ SmolGui ★]

As asked above. Completed the War Within a while ago and noticed that Kuva siphons have been appearing on most planets. What do you actually do with Kuva?

[Sceles]

Mainly for rerolling rivens. Some items also have it as a crafting requirement.

[William]

and for crafting component

[Archwraith]

Roll riven mod stats, materials to craft Zarr, Kesheg, Twin Rogga, Orvius, Harrow, Nidus, Octavia part

[Totally Innocent Chatbot]

A couple items have it as a crafting component, such as the Zarr cannon and the frame Nidus. But mostly, it's used as the currency for "rerolling" Riven mods, which generates a new set of stats for the mod (but does not affect what weapon the Riven can be used on or the Riven's Mastery Rank requirement or polarity), then lets you decide whether you'd rather keep the new stats or the stats the Riven already had.