Why did they erect the rocks in Gilgal?
Rashi: It is the Malon (place where they will lodge); they were commanded to erect the rocks there. 1
Radak (19): They traveled more than 60 Mil that day, including women and children. This was a miracle (normally, a person can go only 40 Mil in a day. And part of the day they crossed the Yarden, gave Brachos and curses, wrote the Torah in 70 languages
Verse 8 says that they placed the rocks in Gilgal. Here it says that they were erected!
Malbim: Verse 8 says that the ones who carried them placed them down without arranging them for a sign for that generation. Here it says that Yehoshua erected them to a height, for a remembrance for later generations.