Why did the brothers repeat the fact that they had come to purchase food?
Oznayim la'Torah: Refer to 42:9:3:6. They were insinuating that they really did come to Egypt to purchase food, and that the reason that they volunteered the information without being asked was due to their state of hunger.
Ohr ha'Chayim: Even if we had no reason to come, there is no reason to suspect us. Further, it is clear why we came - to buy food!
Malbim: There are four reasons to suspect them of being spies. (a) If we had no reason to come. It is clear why we came - to buy food! (b) If different families send spies, since each seeks different matters (pasture, or fertile farmland), they need to send many spies. We are from one family - if we were spies, one or two would suffice! (c) Even from one family, they might send many spies to investigate different places in a big land. We are Kenim - just like we are together now, we have been together since we entered 1 ! (d) If it is known that they were once spies, then we can suspect that also now, they are spies. We were never spies!
This is unlike Rashi, who explains that each entered a different gate. (PF)
QUESTIONS ON RASHI
Rashi writes: "'Lo, Adoni' - [No, my master,] do not say thus!" Why explain this way, rather than simply as, 'It is not as you said, [my master]'?
Gur Aryeh: If they were referring the viceroy's words, they should have said "Lo Ken Adoni" - it is not so.
Rashi writes: "... she'Harei (for behold), your servants came to buy food." Why does Rashi insert this one word into his explanation of the Pasuk?
Gur Aryeh: The verse reads, "It is not as you say... and your servants came to buy food." This appears to be a two separate statements, making the conjunctive letter Vav ("and") difficult to explain. Rashi inserts the word "Harei," meaning, 'but behold! We came to buy food.'
Rashi writes: "... For behold, your servants came to buy food." This proves nothing! Perhaps they were buying food only as a cover for their true mission as spies?
Gur Aryeh: In fact, they were not trying to prove their innocence (i.e. that they were not spies), but rather just stating the truth, that they came to buy food.