Music is the reason main reason SOME groups are successful, in other cases music is PART of the reason they are successful, and in some cases (NMixx) groups are somewhat successful DESPITE the music.
Like it's not one rule for all groups, groups have popularity and success for different reasons.
Groups successful with the main reason being music (or music-related stuff, such as performances). Examples -
NewJeans
BTS
(G)I-DLE
Ateez
Mamamoo
Oh My Girl
StayC
The above groups are mostly non big-4 (or non big-3 when it was just that) and have generally had music and/or performances, and a bit of a hard slog to grow where they are. For some of them, their album growth from debut to latest release has multiplied by triple digits (ie 100x, 200x). It's phenomenal growth. For others such as StayC and NewJeans, they've primarily hit big due to music. Yes sometimes some backing, but the music is still what catapulted hitherto unknowns to national attention.
Groups successful with music (or were), but also with a large portion of other factors (previous interest, celebrity virality, visuals, label and/or combination of those factors). Examples -
IVE
Le Sserafim
Aespa
Itzy
Kep1er
Txt
NCT
These groups have had strong songs and great success mostly, but also have debuted with either in-built fandoms, or general awareness, knowledge, and visibility denied to many groups. No-ones doubting, for example Love Dive was a good song, but people know of Wonyoung previously, and she helped it regain it's spot and longevity after a viral appearance by her at a baseball game. Many of the others on this list came either with in-built fandoms or wide visibility, and continued wide visibility, due to non-musical factors. This is a key component to why they have succeeded, but of course the heights of that success is helped by music, it's just not the only primary component.
So TLDR - it can be both mainly music or only partly music. Different reasons why some groups made it big. Each success feeds on itself and generally maintains that success, but there are different ways these groups sold 100k+ albums.
