Yes, lots of little things added that make up to a big difference:
Bloom lighting
God Rays (Volumetric Lighting)
DoF
Ambient Occlusion
SMAA Injector (very similar results to MSAA at a fraction of the performance loss)
Sub surface scattering
HDR Lighting
Parallax textures
Custom Anisotropic filtering
Custom colour palette
Improved specular reflections
Improved shadow detail
All these can be adjusted to with either the in game pop up menu for real-time editing or with a text editor as the ENB file settings are very simlar to an .ini file.
Personally I can't play Skyrim without one, the game's narrative just isn't strong enough to suck me in and the default lighting and graphics engine is the very definion of "Vanilla." Running an ENB helps to create an atmosphere that immerses me a bit more (along with a lot of other mods) and lets me make Skyrim look like what I think of as a fantasy environment. My favourite are the Grim & Sombre ENBs as they convey an opressive atmosphere for an area that is meant to be in the middle of a civil war. They are quite intensive and many come with a performance version with the settings turned down so that the game is actually playable.
Not sure how much difference an ENB will make in GTA as the game already has many of the features above but at the very least it would give you the chance to tune the game to your specific preferences.
Check out Hodilton, who does a lot of mod videos on ENB's and other graphical mods.
https://www.youtube.com/watch?v=HMEWkJ-1LBs