What is the function of T’naim when performed just before the chuppah?
As heard on some yutorah lectures:
A more traditional Tnaim was a very strong contract, with stiff penalties, that the couple go forward with their marriage. Originally it was done around the time of engagement. Over time, people were concerned that the engagement might not work out, so they delayed the tnaim a bit. Then a bit more. Then a bit more. Eventually they were doing it right before the wedding, which made no sense.
The late Rabbi Moshe Feinstein reformulated the document, for use right before the wedding. It states:
It could be used in case the day after the wedding, the father-in-law says "hey wait, I was supposed to be paid $1000 if this shidduch went through!" or something like that; it says black on white that all claims have been settled. And it's an important emotional statement; and a nice reminder about the couple sharing (and their parents pulling back so the couple can be a couple!) and not sheltering assets. But a lot of it was because people were used to having some sort of Tnaim document; once they were going to have one, this formulation made the most sense for the way it was being used.
T'naim directly before the chuppah serves the exact same function as T'naim a year/month/day/century before the chupah.
The reason why a T'naim is done so close to the chuppah (in some circles) is so that there is little to no chance of the T'naim being broken. Unlike a wedding or Ketubah, a T'naim can not be resolved with a get or similar document, but must be fulfilled or a Beit Din must be found to annul it (Which I hear) is almost impossible to do.