The Gemara on 173b first asks what the source for the shibud of an arev is and as the rashbam explains, that the question is how the arev is meshabed himself with amirah b'alma and the gemara then brings several pesukim to answer the question. however, a few lines later the gemara says that an arev is not like one who makes an asmachta because of the hanah that he has when the malve believes him and the rashbam adds that the arev is making the malve into his shliach to give the loan to the loveh,
WHY IS THE RASHBAM ADDING THIS, IT SOUNDS LIKE HE IS EXPLAINING HOW THE DIN OF AN AREV WORKS AS A KINYAN, YET THE GEMARA ALREADY ESTABLISHED THROUGH PESUKIM THAT AN AREV IS MESHABED HIMSELF WITH AMIRA B'ALMA EVEN WITHOUT A KINYAN??!!
Menachem Obadia, Miami Beach, US
While it is true that the Gemara already established this from Pesukim, the Gemara did not establish what the reasoning is behind the law. In monetary laws, we do not generally have laws without any apparent reason. This is why Rav Ashi states that the reason that the Pesukim state that this works, and it is unlike regular words that are not binding, he in fact is Meshabed himself due to the benefit that he receives. Rav Ashi is explaining why this works, and the Rashbam just explains Rav Ashi's reasoning further.
All the best,