You get longer playtime than a 7" single. 12" singles were only ever marketed for extended remixes - longer play time than a 7" single but cheaper than a 33 1/3 RPM album. Do you have any ads for 12" singles that talk about better sound quality?
Okay, maybe some unintuitive pizza math works here.
7" 45 rpm record, about 4.5 minutes max. Center label is 4". Label area = pi * 4^2 = 50.27 in^2. Total record area = pi * 7^2 = 153.94 in^2. Playable area (max, ignoring runout and edge) = 153.94 - 50.27 = 103.67 in^2. Call it 100 in^2.
12" 33 rpm single, about 12 minutes max. Center label is 4". Label area = 50.27 in^2. Total record area = pi * 12^2 = 452.39 in^2. Playable area = 452.39 - 50.27 = 402.12 in^2. Call it 400 in^2.
So, the 12" record has 400/100 = 4x as much space, but the playtime is 12 / 4.5 = 2.7x. For the remainder, you get quality. If the 7" record was 33rpm and you had about 7 minutes max, then 12/7 = 1.7x, which means even more room for quality improvements.
In general, you can trade play time for quality when cutting vinyl. If your grooves are at the maximum width, to the point that you'd leave substantial uncut vinyl between them to fill up the track width on your record, then you can get even more quality by moving from 33rpm to 45rpm because now a longer length of groove contains the same length of sound. All else considered, 12" 7-minute 45rpm sounds very good.
And also, I know the original reference was to disco, but new 12" singles are still being released, sometimes exclusively on vinyl. It slowed down, but it never stopped. Sorry, no ads, just personal experience.
I've never seen an ad for one. Most of mine were distributed to DJs only. Another nicety with the DJ ones is they didn't have pops and clicks when new. Sometimes the flip side was the same edit - in case you damaged one side, use the other!