i think he needs it to fight madara,his super speed only works for a short distance,plus we saw him use that speed 3 times only,plus the fact thaat minato sealed kurama inside naruto to complete "that jutsu" which has a goal to defeat tobi,please don't say "that jutsu" is TBB because it's useless against obito,minato wanted something that makes naruto capable of fighting tobi,which is none but FTG