This is also one of the questions that has its answer right in the text, namely in the Leviticus 24:9.
9 And it shall be Aaron's and his sons'; and they shall eat it in the holy place: for it is most holy unto him of the offerings of the LORD made by fire by a perpetual statute.
9 וְהָֽיְתָה֙ לְאַהֲרֹ֣ן וּלְבָנָ֔יו וַאֲכָלֻ֖הוּ בְּמָק֣וֹם קָדֹ֑שׁ כִּ֡י קֹדֶשׁ֩ קָֽדָשִׁ֨ים ה֥וּא ל֛וֹ מֵאִשֵּׁ֥י יְהוָ֖ה חָק־עוֹלָֽם׃ ס
So, as you can see, these twelve Shewbreads served as a special Tabernacle "snack", which Aaron and his sons would eat when they were performing their priestly duties in the Tabernacle.
It is also important to point out that the previous verse (Leviticus 24:8) speaks that these twelve loafs of Shewbread were to be renewed with the fresh ones every Shabbath (every 7 days). This means that there were 12/6=2 loafs of Shewbread dedicated for every day of the week, except Shabbath, when there were twice as many loafs (remaining old ones and the fresh new ones).
Also, it is quite obvious that each loaf of Shewbread symbolized one of the Tribes of the Children of Israel.