Why did it need to say "l'Binyamin"? It would suffices to say "l'Ish Boshes", like it says "me'Avdei David"!
Radak: This teaches that they were from Shevet Binyamin, and were going with him to declare him king.
Malbim: There were 12 men, because Ish Boshes had 12 Shevatim 1 under him, with Binyamin at their head.
I.e. he counts Efrayim and Menasheh separately, and also Levi. (PF)