Malbim citing Mahari: It was not in order to destroy the wall of Yerushalayim. Rather, it was for his honor. This is the custom of the Roman emperors. When they have a great victory in war, they do not enter the city via the opening. Rather, they breach part of the wall in order to enter via the breach, to testify that they made a great breach in their enemies. So Yo'ash did to publicize his victory.

Radak: There was a gate in Yerushalayim called Sha'ar Efrayim. It was open, opposite the border of Efrayim, and through that gate Bnei Efrayim entered Yerushalayim.

Radak: It is "Sha'ar ha'Panim" (Zecharyah 14:10). It is called so because there was the Pinah (corner) of the wall.