Why do slicers have nozzle size settings?
I never thought about that! 

I do know from experience that Cura uses nozzle sizes to sanity check your settings, warning you if you do something unreasonable. That is reason enough to include the feature for me. However, I did wonder what, if any, differenced this actually makes, so I tested it and here's my results: 

I went through and tested it on the 3DVerkstan Face Mask. I compiled the same model in the same place in Cura, the only differences between the two being that I changed the nozzle size from 0.4&nbsp;mm to 0.8&nbsp;mm. I then ran the VS Code Diff Checker on the files as well as loaded the exported gcode back into Cura to compare and got some interesting results. 


The estimated times and filament usages are very slightly different: 2689 seconds for 0.8 and 2698 seconds for 0.4, 6.18899&nbsp;m for 0.8 and 6.18883&nbsp;m for 0.4. This means something different is happening somewhere, but it's very similar to within a rounding error. 
The brim is exactly the same for both. No differences whatsoever in the G-code.
When the model starts printing, everything in the gcode becomes different. This seems to be down to a few reasons: (note these photos were taken by exporting gcode then re-opening that gcode into Cura)


Differing layer start positions

Possibly a difference in line overlaps? On the 0.8&nbsp;mm file, cura shows a larger gap between the brim and the part, and more overlap between the outer wall and inner walls when compared to the 0.4&nbsp;mm file. 




That's all the differences I can observe in this model. Every layer is different in the gocde, but it seems to be down to the two differences noted above. IMO, that's all inconsequential and you should be fine to change all the settings to how they should be for a larger nozzle and be just fine. Other than the warnings, nozzle sizes may be a holdover from a past version when they were more useful, or simply may have just seemed like a reasonable feature to put in at the time and no one really thought about it. 



As far as changing nozzle size goes, I go to Preferences > Configure Cura > Printers > Machine Settings > Extruder 1 > Nozzle Size. If that option is not there, consider creating a new machine of a different type that's similar to what you have. I have an Anet-A8 but use the Prusa i3 printer type in Cura since that's most similar to what I have.