Abinadi had unsuccessfully preached to the people in Lehi-Nephi only two years before. He is called to preach to them again. In this occasion, he comes in disguise. Then, the first thing we see him saying is “the Lord commanded me, saying—Abinadi”! Of what value is a disguise if you identify yourself at the first opportunity?
There is no answer given in the text, so we are left to supposition. Since Abinadi had preached before, it was possible that he might have been recognized. Perhaps he would not have been allowed into the city. Whatever the case, what Abinadi does is enter the city in disguise and put himself in a very public place. It is in that public place that he begins to preach and make his accusations.
Since it was the Lord who had commanded that Abinadi preach, the Lord certainly understood what the result of that preaching would be. From the standpoint of making a difference in the lives of the people of Lehi-Nephi, Abinadi would make only a single convert. However, that single convert would be Alma the Elder, and Alma the Elder’s contributions to the rest of Nephite history would be significant.
Therefore, we may surmise that the Lord, knowing the importance of that one convert, needed to place Abinadi in a position where he would speak before the priests of Noah, one of whom was that very Alma. Identifying himself in a public place probably assured that he would be taken to be tried before King Noah and the judges. Once Abinadi had entered the city, there was no longer a need for the disguise because it was the goal to stand before the king and his priests.