Why does the Pasuk insert the words "ve'Chi Yirbeh Mimcha ha'Derech"?
Rashi (Pesachim, 75b): To teach us that one is not permitted to redeem Ma'aser Sheini in Yerushalayim.
Even to eat it bi'Kedushah.
What are the implications of "Ki Yevarech'cha Hashem Elokecha"?
Rashi and Ramban: It implies that there will so much produce that (in conjunction with the distance involved - Ramban) it will be too much to carry to Yerushalayim.
And the Torah is therefore providing the option of redeeming it and taking the money to Yerushalayim in its place.