It ties up the loose end of a plot device
"We gotta discourage this sort of thing. Ya follow?"
The major plot device of the film is that Lonnegan is a dangerous man that wants the grifters that suckered him out of $11,000 removed before anyone else finds out. That he manages to get Luther killed fuels the rest of the plot. We would not have a film without that as a driving force; Luther would retire, and Hooker would just become a grumpy student of Gondorff's (maybe). So Lonnegan's contract on them is critical to the entire plot.
But, what then to do with this plot device, once it has served its purpose? We cannot just leave it dangling.
If we were to speculate that Lonnegan's hitmen simply failed to track Hooker, and nothing more ever happened in that regard, the audience would begin to wonder what happened with Lonnegan's revenge, and ask uncomfortable questions. It would be a "What Happened To The Mouse" kind of annoyance.
David Ward instead uses this plot device to create nice side-plots as padding, tension, and even a low-key action sequence, ending in a delightful mini-twist before the big showdown.
So, could the plot have worked without Riley, Cole, and Salino?
Yes, it could have. But it would not have been the Best Film at the Academy Awards, since it would have felt incomplete, like the plot device got sloppily discarded, and also losing some of the nice tension that results from Hooker having people on his tail the whole film.