Searching the CuraEngine source, the only places I can find where nozzle size is used directly involve some arcane logic for merging of infill lines, such as:
So essentially, yes, nozzle size mainly serves as a default value for the line width settings.
However, in general it's not always possible for line width different from nozzle size to be honored.
For line widths smaller than the nozzle, motion along a path where the material being deposited has proper adhesion is required to stretch the material and prevent it from retaining the width extruded from the nozzle, and small lines already printed are likely to be damaged by the nozzle when attempting to print additional small lines in their vicinity. In particular, you won't be able to print small details significantly below the nozzle size just by extruding less material.
For line widths larger than the nozzle, compression against an existing surface is required to prevent the extra material from just sagging down rather than expanding horizontally. In particular, line widths wider than the nozzle are unlikely to work right in the presence of overhangs.
Also, as noted by Tim Kuipers in comments, there are places outside the source, but rather in the json-based configuration tree, where nozzle size plays a role in the defaults and warning ranges for other parameters. Those are mostly line widths, but
The nozzle size can affect the values of the following settings besides line width in
fdmprinter.def.json: Outer Wall Inset, Outer Wall Wipe Distance and Minimum Support XY Distance. Other than that it only influences the conditions under which setting-values give you a warning.