Radak citing Targum Yonasan: He is honored and great in prophecy; "everything that he says comes [true]." Even though this is true of all Nevi'im, he needed to say so, for Nevi'im were rare then until Shmuel had prophecy. Astrologers, witches or Ba'alei Ov, if they say about the future or what is hidden, often it is false.

Malbim: It is a G-dly person who is truly esteemed. However, not all of them can tell the future or what is hidden. We need one that "everything that he says comes [true]." It did not suffice to say the latter, for there were also witches who told the future. They are not honorable, and one who goes to them, he is disgraced.

Rashi: The episode of the donkeys - why we had this [long] journey for them.