YouTube Vanced was so well-liked because it offered premium-like functionality for free, which included:

Ad-Free Experience – Blocked all YouTube ads.
Background Playback – Supported videos playing in the background while other apps were used.
Picture-in-Picture (PiP) – Supported floating windows of videos.
SponsorBlock Integration – Skipped sponsored parts of videos.
Dark and AMOLED Themes – Provided improved viewing for low light.
Override Video Resolution – Forced high-quality play no matter what the network.