What is the role of a monochromator in AAS?
This question has lingered on the site for a long time, which is a shame because it is a good question to ask.  Atomic absorption spectroscopy (AAS) uses a hollow cathode lamp (HCL) as a source, and HCLs are line sources; therefore, if a line source is being used, why would an instrument need to be made more complex by adding a wavelength selector?

The first point to consider is what the source really looks like.  The emission spectrum from HCLs have multiple lines.  Here, for example, is the emission from a nickel HCL, taken from Spectrochemical Analysis by Atomic Absorption and Emission by L. Lajunen.



A wavelength selector is needed in order to ensure that only one of the emission lines is being used for the analysis.  

Additionally, light can be scattered in the flame, which can introduce errors in the analysis if not eliminated.  If you compare the block diagram of an AAS vs a typical visible spectrometer, you'll note that the wavelength selector is typically placed after the sample compartment for AAS to address these concerns about scattered light; however the issue is not that significant in visible spectroscopy so the wavelength selector can be placed before the sample holder.

This representative block diagram for AAS is taken from here.



A representative UV/Vis spectrometer block diagram is from this source



Bottom Line AAS needs a wavelength selector to ensure that the analytical line of interest is the only radiation hitting the detector and it minimizes the negative effects of radiation scattered in the flame.  A good summary can be found here.