This is a rather specific question not of language syntax but rather of its origin/history. I’m just trying to understand the specific reasons behind the need for adjective declension.

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> I have read somewhere that, it is done to indicate the case and differentiate singular vs plural.
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However I'm not sure if this holds true. Let me explain with a few examples.

* **Nominative**:

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> Die blaue Hose ist groß. (Singular)
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> Die blauen Hosen sind groß. (Plural)
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Now, as we can seen *blau* is declined in this case differently for singular and plural.
The distinction in singular and plural is due to the noun’s ending as we can see (*Hose* vs. *Hosen*), and since the verb *sein* is used, we know that it is nominative. So why even bother such declension?
* Similarly, in the **accusative** case:

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> Ich habe eine blaue Hose. (Singular)
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> Ich habe die zwei blauen Hosen. (Plural)
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Here also, we know *haben* is an accusative verb, so case is indicated through it and the noun differentiates singular and plural. So why do we need to do it?

In nominative and accusative cases, there is no consistency of declension while in dative case there is mostly a common *-en* ending.

Can someone explain to me why exactly we do adjective declension? Is there any specific role behind it or is it just an accepted norm?

I’m not being judgemental or anything; I am just curious, in particular because it is so different in English (which has the same origin as German).