ETA: but we don't know why Quirrel would do that; see discussion below.

Misdirection? The scenario would go like this: ideally, Bella is found dead, and chucked out as just another Azkaban victim.

In the event anyone is suspicious and does some checking, the easiest thing to do would be to inspect the contents of the cell, immediately uncovering the poison. (If you were suspicious, it would take you a long time to work down the list of suspiciousness to 'a genius wizard used an incredibly advanced spell to create an identical dead body'.)

Instantly, the investigating Auror's mind deduces that someone good wanted Bella dead, and unable or unwilling to use the Killing Curse (stealth), used an alternative method (poison).

This is even wronger a theory than just being suspicious, and it's completely contradictory to a escape theory (because the investigator has to postulate someone on the level above seeking to manipulate them before the presence of poison has an explanation), so it's even less likely anyone will wonder whether it's the real Bellatrix and check for the dummy.