Nothing really. I should probably remove the option.
For a long time I used the default of 1536MB VRAM on my 2017 HP Spectre X360 laptop which has HD620 IGPU driving a 4K LVDS panel (3840 x 2160) quite often i would see slight glitches with the display when fast scrolling long web-pages and playing high res YouTube videos. The glitches are minor and do not really cause any problems other than a brief cosmetic one.
A few weeks ago, as an experiment i tried the 2048MB VRAM patch via HD-Patcher & Device Properties, like you i really did not think it would make any difference but since implementing the patch i have not seen any glitches .... so I think that extra bit of VRAM does help if driving high resolution 4K displays ...
The system is running Mojave 10.14.1 ... Which means the GUI is rendered via the Metal 2 API ... everything in the MacOS UI seems to be just that little bit smother with the patch .. especially the dock and LaunchPad .. I wonder if Metal 2 uses all available VRAM more optimally then previous MacOS versions that used Open GL/CL ?
This morning after reading your post, I disabled the 2048MB patch and went back to 1536MB, sure enough the screen glitches where back when fast scrolling in chrome and playing YouTube videos, after re-enabling it they where gone.
Based on my findings I would say keep the 2048MB option in FB-Patcher, i was thinking of adding it to my guide
Interestingly (or not) the laptop also has a Nvidia GTX 940MX DGPU (disabled in MacOS) that appears to be used most of the time when running Windows 10, it has 2GB of dedicated DDR5 VRAM so maybe 2GB VRAM is the sweet spot for desktop GUI's on 4K displays ?
Keep up the great work