Nauka should, hopefully this year, replace the ISS' Pirs module.
While Nauka is already pretty old, I suspect it will bring a lot of interesting features to the space station. However, Pirs might have some unique capability/features that might not be present in Nauka.
What would they be?
The DC-1 module (PR name Pirs) provides a crew airlock and a docking port.
The MLM module (PR name Nauka) will not provide a crew airlock. It will provide a scientific airlock along with many other capabilities. It does have a docking port.
The MRM-2 module (PR name Poisk) will still provide the Russian side of the ISS with a crew airlock.
