Why use a let down plate?

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Why opt to use a let down plate? [title amended for clarification]

I've done quite a few ILS approaches. Although I obviously had to do an NDB / DME approach procedure for my IMCr test, I've never actually come across a real "live" situation where I have been told to, or requested the procedure.

So, my question is probably very simple and it is, under what circumstances might you OPT to request (or be TOLD to use) the procedure rather than asking for vectors to the ILS? (obviously its the only choice where there is no ILS).

thanks.

[OP amended for clarity!]