If an artist is still recording music when their "Greatest Hits" or "Best Of" release comes out, often the artist will include one or two new songs to entice buyers. This has been true for decades. It's only when the artist is no longer recording that it may be more of a true "Greatest Hits" type of package. But even then, sometimes a new mix will be tossed in (like a recent Elvis "Best Of" of a few years ago) or the songs will be focused on a theme (like the Beatles' "1"). But in general, artists include some new release in the "Best Of" to get fans to buy yet another album.
Hence, the Green Day song and the new U2 song are the "reasons" for fans to buy this "Best Of the Best Of", even though it's premature and superfluous.