The Gemara in Bava Basra (40b) discusses the law of the Modaah, and says:
דלא כתיב בה אנן ידעינן ביה באונסא דפלניא לאו מודעא היא מודעא דמאי אי דגיטא ודמתנתא גלויי מילתא בעלמא היא ואי דזביני והאמר רבא לא כתבינן מודעא אזביני לעולם דזביני מודי רבא היכא דאניס וכמעשה דפרדיסא דההוא גברא דמשכין פרדיסא לחבריה לתלת שנין בתר דאכלה תלת שני חזקה אמר אי מזבנת לי מוטב ואי לא כבישנא לשטר משכנתא ואמינא לקוחה היא בידי כה"ג כתבינן מודעא:
that does not have written in it the formulation: We are aware of so-and-so’s duress, i.e., we are aware of the nature of the coercion that forced him to enter this arrangement against his will, is not a valid preemptive declaration. For what type of transaction is the preemptive declaration being stated? If one were to say that it is a preemptive declaration for a bill of divorce or for a gift, the preemptive declaration is merely revealing the matter. Since these actions can’t take place unless he desires it, it is sufficient that he stated that he does not desire them, and he need not specify a particular reason for nullifying them. And if it is for a sale, but doesn’t Rava say: We do not write a preemptive declaration for a sale? The Gemara answers: Actually, it is referring to a preemptive declaration for a sale, as Rava concedes in a case where one was compelled to act due to a threat of monetary loss, as with the incident of the orchard, as there was a certain man who mortgaged his orchard to another for three years. After he worked and profited from it for the three years necessary for establishing the presumption of ownership, he said: If you sell the orchard to me, it is well. And if not, then I will hide the mortgage document and I will say that this land is purchased and that is why it is in my possession, and you will receive no payment for the orchard. In a case like this, we write a preemptive declaration. The declaration states that he does not actually desire to sell his property but was forced to do so.
It looks like the point is to cancel a fishy sale in advance.
But what's the point?
If such sales shouldn't be valid, then make them invalid. And if you can cancel a Modaah, then any robber worth his salt will just force him to write a Bittul Modaah before the contract.