Why do we need the second reason?

Question

Chagigah 3:8:

כָּל הַכֵּלִים שֶׁהָיוּ בַמִּקְדָּשׁ, טְעוּנִין טְבִילָה, חוּץ מִמִּזְבַּח הַזָּהָב וּמִזְבַּח הַנְּחֹשֶׁת, מִפְּנֵי שֶׁהֵן כַּקַּרְקַע, דִּבְרֵי רַבִּי אֱלִיעֶזֶר. וַחֲכָמִים אוֹמְרִים, מִפְּנֵי שֶׁהֵן מְצֻפִּין:

All the utensils that were in the veins hamikdash need immersion (after a regel, because of the amei ha’aretz that touched them), except for the golden mizbeach and the copper mizbeach, because they are connected to the ground, according to rabbi Eliezer. The Chachamim say, because they are coated (in metal).

1. Why did the Chachamim feel the need to add their own reason why we don’t immerse the mizbe’ach? What does it teach us?
2. Why does the metal coating affect the status of the mizbe’ach, according to the Chachamim?

Answer 1

Chagigah 27a gives two explanations of the Sages' opinion:

1. They are actually arguing against R. Eliezer. They believe that the altars are susceptible to impurity, and are not viewed like the ground, because they are coated with metal.

2. They are saying the following to R. Eliezer: "Why do you need to compare the altars to the ground in order to render them free from impurity? Because they are coated in metal? The metal coating is irrelevant, thus they are simply wooden, immovable vessels which cannot become impure, and there is no need to compare them to the ground in order to render them not susceptible to impurity."