In the musaf for Shabbat we make the following plea about the future (loose translation mine):
שֶׁתַּעֲלֵנוּ בְשמְחָה לְאַרְצֵנוּ. וְתִטָּעֵנוּ בִּגְבוּלֵנוּ וְשָׁם נַעֲשה לְפָנֶיךָ אֶת קָרְבְּנות חובותֵינוּ. תְּמִידִים כְּסִדְרָם וּמוּסָפִים כְּהִלְכָתָם
That you will raise us up in joy to our land, and plant us with our borders, and there we will perform before you our obligatory sacrifices - the orders of the Tamid offerings and the additional offerings as per their laws.
Then the text continues:
וְאֶת מוּסַף יום הַשַּׁבָּת הַזֶּה. נַעֲשה וְנַקְרִיב לְפָנֶיךָ בְּאַהֲבָה כְּמִצְוַת רְצונֶךָ
and the additional offerings of this Shabbat we will perform and sacrifice before you with love, as the [performing the] commandment of your will
I can't figure out why we include this line.
We have just said that we will perform the musafim -- so why do we say "and we will offer the Shabbat musaf"? If I say "I'm going to eat all the food" I don't then need to say "and the bread" because bread is included in "food." The text could just jump to כְּמו שֶׁכָּתַבְתָּ עָלֵינוּ (though IMHO the entire paragraph should be re-ordered with the pasuk coming after וַתְּצַוֵּנוּ ה' אֱלהֵינוּ לְהַקְרִיב בָּהּ קָרְבַּן מוּסַף שַׁבָּת כָּרָאוּי and then the wish for the future appended).
This structure, by the way, is the same for Shabbat-R"C and for holidays. We pray for the future, commit to performing all the musafim and then add on the specific one which we will also do.
Originally, I thought that the point was that we will offer (at least) the Shabbat musaf with love, but then I saw that the pasuk quoted supports just THAT we are commanded to offer it, not how we will or should do it, so I am still at a loss.
Yes, this could just be a matter of a human text writer trying to be poetic and nothing need be read into the specific choice of text, but is there any authority who explains a value to the additional restatement of our intent to perform the musaf of the day?