I've never read anything by Galgut, so I can't say whether his recent work The Promise is Booker-worthy or even good. However, my broader experience with book awards gives a telling look into the world of prizewinning reads.
During lockdown, I decided to use the list of Hugo Award nominees and winners to direct my reading. The Hugo Awards are named for Hugo Gernsback, publisher of the first ever science fiction magazine. While early nominees were mostly science fiction, modern nominees come from all genres and subgenres of speculative fiction. The Nebula, Locus and World Fantasy Awards are similar.
Having read some of the winning novels, novellas, novellettes and short stories, the award has brought to my attention works I wouldn't have otherwise read. For instance, I wouldn't have read the short fiction of Gene Wolfe and Connie Willis had they not received multiple nominations each for 'Best Short Story' and 'Best Novella'.
That said, there is a lot of great fiction that isn't considered for these awards. I think these things too often put too much emphasis on the 'best' works. As an avid reader, I'm less interested in one or two 'best' works and more interested in a variety of works that are good in different ways. This is why I think the whole array of nominees is more interesting than just the winners.
There's also the question of who makes the decisions. While the Hugo Award takes nominations of anything members of the World Science Fiction Convention have read, heard or watched, other awards often pick from a narrower selection. Even the 'anything WSFC members enjoyed' concept will be biased by who the members are and where their tastes lie.
As an avid reader, I'm less interested in one or two 'best' works and more interested in a variety of works that are good in different ways.
Although women have been eligible to be nominated from the start in 1956, most early Hugo winners were men. (Ursula K. Le Guin was the first female winner for Best Novel in 1970.) The prevalence of men was so strong that some members believed the increase in female nominees in the 2010s was based on unfair favoritism. While there's no way to prove why nominees were chosen, this could easily be explained by a recent increase in successful female authors or current members taking a greater interest in books by women than past members. Other awards have faced similar issues when people are upset about who should or shouldn't receive awards, because these contests can intentionally or unintentionally create a sense of tradition or old standards that should be upheld.
This brings me to the central issue I see with book awards. Highlighting a few of the 'best' always relies on a few people basing judgment on their own opinion or whatever is most popular amongst those with the loudest voice. What makes a book one of 'the best' anyway when everyone will have a different view of what that means?
Furthermore, promoting 'the best' books suggests to audiences that this is what they should be reading. The notion of 'the best' implies to the casual reader that the rest of the genre isn't worth looking at. The focus on 'the best' works also discourages audiences from seeking out what they personally enjoy and works by lesser-known authors.
From a creator's perspective, I think awards have certainly highlighted the work of many talented authors and that authors deserve recognition for their hard work and creativity. However, as a reader, I believe that awarded works are merely a starting point for a genre or an author's bibliography, not the only works worth reading. It's time to stop framing them as such.