sentence construction - what does 'all the more significant for being so rare' mean?

Excerpted from the site sparknote:

Amidst the callousness and exploitation, moments of compassion are few and far between, although perhaps all the more significant for being so rare. Even though he is taken in only grudgingly, the old man eventually becomes part of Pelayo and Elisenda’s household. By the time the old man finally flies into the sunset, Elisenda, for all her fussing,

I couldn't make heads or tails of the bold part not only semantically also analyctically to analyse the part(grammatically).

Answer

Original text:

Amidst the callousness and exploitation, moments of compassion are few and far between, although perhaps all the more significant for being so rare.

In plain English:

Being surrounded by the emotionless and unfair treatment, moments of compassion are rare, but perhaps these moment are even more significant because they are very rare.

---

Let's take a closer look at the transformation of the part you ask about:

although [ perhaps ] [ all the more significant [ for being so rare ] ]
= but [ perhaps ] [ these moments are even more significant [ because they are very rare ] ]

Explanation:

although = but. In formal writing we can use although to introduce a reduced clause (a clause without a verb). The reduced part in this context is these moments.

all the more = even more.

for being: for in this context is used to mean because; for being so rare in this sentence means "because they (moments of compassion) are so rare".