It’s valuable if you don’t have a good handle on what you are looking for. Like if you knew the word poker but didn’t know what a “poker chip” was. Bad example, but the point is, it shows the most common searches. This is very valuable to webmasters because you can see all sorts of keywords that never occured to you that are not just popular, but the most popular given your first words. For example, type in “online casino bonus” (no quotes). Five things are shown, three are obvious, but the other two? I wouldn’t have guessed those.
A more obvious example is if you type in blackjack. See the common two word searches.
If ever added to the main search it would obviously lead to much more competition for searches shown for common first words… and it would lead to “serp bombing” where bots type in relatively unknown variations that the bomber ranks first for, like say “blackjack now play online”. Real people don’t search for that exact phrase, but if you bomb it to be second under blackjack, people would likely click it.