What is the purpose of всё in this sentence? Я всё ещё читаю этот роман
In this particular case "всё ещё" together mean "still". As in "I am still reading this novel."
Now, there is a catch: "я ещё читаю этот роман" also translates into "I am still reading this novel."
The difference is in the emphasis. "Ещё" on its own just means continuation of the action, the fact that it hasn't completed yet.
Combined "всё ещё" has an additional connotation of process taking longer than originally expected.
всё ещё нареч. обстоят. времени До сих пор, пока что.
Все еше is an adverb of time which means "still" like еще, but with more emphasis (все). It shows that the action is still in process and the person has spent some time reading.
Я все еще читаю этот роман.(I am still reading this novel and I have been reading it for a long time