Why does the Torah add "Ki vo Bachar Hashem"?
Seforno: 'It is befitting that you provide the Kohen with food and clothes to enable him to stand and serve before Hashem'.
Why does the Torah insert the word "La'amod Le'shareis"?
Rashi #1: "La'amod" teaches us that Kohanim must perform the Avodah standing (and we learn from "ke'Chol Echav ha'Kohanim ha'Omdim Sham"in Pasuk 7 that it is crucial to the Avodah - Zevachim, 23b). 1
Rashi #2 (Yoma, 25a): "La'amod Le'shareis" and "ha'Omdim Sham" (in Pasuk 7) teaches us that sitting in the Azarah is prohibitted.
Chulin, 137a: We learn from "Leshareis" that Reishis ha'Gez only applies to the wool of sheep, which is fit for the Avodah - in the form of Techeiles in the Bigdei Kehunah - to preclude goats'-hair. Consequently, any Korban whose blood the Kohen received in a bowl sitting is Pasul. 2
Chulin, 138a: We learn from "Leshareis"that the minimum amount of wool that one should give the Kohen is enough to make a belt for the Bigdei Kehunah, for the Avodah. 3
Why does the Torah add the words "Kol ha'Yamim"?
Sifri: To include the Avodah of Chutz la'Aretz - Birchas Kohanm - in the Din of standing. -