1)
Why does it say "Lema'an Yechezku b'Soras Hashem"?
1.
'Rashi': Giving the gifts to them will strengthen their Torah. (a) They will have more desire. (b) If their income is provided, they can engage in Torah. Torah was given only to those who eat manna (they did not need to work for food) - second to them (in ability to learn) are those who eat Terumah.
2.
Malbim: We do not give to idle Kohanim and Leviyim, only to those who engage in Torah.
