Do you think the purpose of education is only the accumulation of knowledge?

Here is the gerund accumulating as a predicative, indicating what the purpose of education is. Gerund is equivalent to noun, which can mean something.

Or you can understand it from another angle:

There is a concept of non-predicate verbs in English. A non-predicate verb is just a word that can not be used as a predicate verb, but can express the meaning of the verb.

A simple sentence can only have one predicate verb, and this sentence means that the purpose of education is only to accumulate knowledge. In this sentence, is is already a predicate verb, so the form of the predicate verb "accumulate" can no longer appear and must be deformed. To do/do/done is the basic form of non-predicate verbs, and do means no occurrence or purpose; Watch-making is happening or active; Done means it has happened or is passive.

Then the relationship between education and accumulation is active, and education accumulates knowledge instead of being accumulated. So the chosen non-predicate verb form is do.

Hope to adopt!