Why User-mode interrupt was introduced in RISC-V?

In RISC-V privileged architecture V1.11 or draft of version 1.12, "N" extension is introduced for User-level interrupts, hardware which implemented this extension can transfer control directly to a user-level trap handler without invoking the outer execution environment.

But there are mirrored CSR and instruction for the N extension, for a system which support M and U mode only, the both save and restore similar context for interrupts, so why the N extension introduced and what benefit can we get?

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