Korver averaged 11 points and 4 rebounds last season for the Hawks on 46 percent shooting, 46 percent from beyond the arc, an absolutely insane number. The Hawks are looking at a bigger rebuild in the event Dwight Howard does not choose them in free agency. (The Hawks are considered a long shot at this point.) Does Korver help that rebuild process? Yes and no.
It's $6 million per year over four years, getting locked up for a 31 year old. It makes their team better, making it harder to tank. However, teams simply always need shooters. They always have value, and Korver is as good as they come. Throw in the fact that Korver was suprisingly good at defense last year (no really) and the Hawks can justify this deal despite their long-term goals.
In the meantime, they get a great shooter who needs to be a bigger part of the offense next season and they get a piece that will still be tradeable down the line. Korver had been linked to the Nets and Bucks among other teams, but had said after the season how much he enjoyed his time in Atlanta.