The line comes from Team Fortress 2, Valve's class based shooter first released in 2007, where teammates can thank the Engineer after his buildings turn a fight. Its clipped, earnest phrasing made it easy to quote outside the game, so it spread through forums, videos and soundboards as a ready made compliment. Players still use it because it works both sincerely and ironically, letting a group celebrate a support player who quietly kept everyone alive, healed and supplied during a match.