Or HaChaim on Numbers 15:38/אור החיים על במדבר ט״ו:ל״ח

1Speak etc. and you shall say: Since there is an element of commandment as well as advantage, and honor which come from the commandment, therefore it said both Speaking (in a commanding form) and saying (in a gentle and respectful form).

2And the shall make for themselves: The reason this passage is connected via the connecting "Vov-and" is as the sages taught, which I wrote about the previous verse, that Moses said to G-d, "During the week your people don phylacteries (tefillin) and thus remember etc. Thus when G-d comes to instruct him that they make fringes (tzitzit), one might mistakenly think that this commandment will take the place of phylacteries (tefillin), that when you wish you may don phylacteries (tefillin) and when you wish you may don fringes (tzitzit) without the phylacteries, even on the week days, implying that either one of these commandments suffices as the reminder about the commandments. Therefore it said, "And they shall make..." implying that the commandment of phylacteries remains and the commandment of fringes is added.