Rashi on Shemos 22:30 says (on לכלב תשליכון אותו):
אַף הַגּוֹי כַּכֶּלֶב; [...] אִם כֵּן מַה תַּ"לֹ לַכֶּלֶב? לְלַמֶּדְךָ שֶׁהַכֶּלֶב נִכְבָּד מִמֶּנּוּ, וְלִמֶּדְךָ הַכָּתוּב שֶׁאֵין הַקָּבָּ"ה מְקַפֵּחַ שְׂכַר כָּל בְּרִיָּה, שֶׁנֶּאֱמַר וּלְכָל בְּנֵי יִשְׂרָאֵל לֹא יֶחֱרַץ כֶּלֶב לְשֹׁנוֹ (שמות י"א), אָמַר הַקָּבָּ"ה תְּנוּ לוֹ שְׂכָרוֹ (מכילתא):
You may give it to a heathen as well as to the dog. [...] But if this be so, what is the force of Scripture expressly saying “to the dog”? It is to teach you that the dog is to be given preference in this respect and Scripture tells you at the same time that God does not withhold the reward due to any of His creatures. The dog is entitled to reward because it is stated, (Exodus 11:7) “But against the children of Israel shall not a dog move its tongue”, and this happened. The Holy One, blessed be He, said: Give it the reward it deserves (Mekhilta d'Rabbi Yishmael 32:30:3).
What does exactly "does not withhold reward" mean?
Granted, if only dogs could get the meat, then this is a reward. Or even if anywhere in halacha it would say that there is a special mitzvah to give it to dogs (like there is a special mitzvah with shiluach haken, or even something like the minhag of feeding birds for Shabbat Beshalach).
But now that Rashi tells us that really there is absolutely no value to giving it to dogs, then what kind of reward do they get?