As a TE, I’ll give my two cents.
From what I have gathered within the past few months, most shows’ presubs are not actually done by Viki’s paid subbers. Usually, Viki’s paid subbers come in when there are shows that have no subtitles. The presubs are generally from the distributor themselves, which can either be translated by people from the distribution company or machine translated, we don’t know.
Now, I have yet to run into a show has been TE’d by a paid subber, but I do know that recent shows that have been coming in are claimed by Viki to already have been TE’d — the messages state that TEs are not needed, or rather, reworded so that it states that the CM should find a general/chief editor only.
But often in these presubbed shows there are still errors.
What Viki’s volunteer TEs do is that they not only correct these errors, but more importantly (in my opinion), provide cultural context that you won’t get on platforms like ■■■■■■■ or the presubs from the distributor themselves.
Example 1:
A character said literally, "You just threw a stone over a wall and killed two people."
What was presubbed/"TE’d" by Viki was that "You killed two birds with one stone."
Now this seems like it makes sense (both have the words "stone" and "killed two"), but what the character was crudely saying was that the person had blindly tossed out a name and it happened to be an actual person in the company. Which is very different from "killing two birds with one stone" which indicates that you were able to achieve more than one thing. This is an example of an incorrect translation made by presubs.
Example 2:
In historical/period dramas, quoting famous people is very common. This typically can be past emperors of China, poets, or ancient texts. These quotes often contain a lot of valuable meaning that adds context to the dialogue. For instance, there was this scene where a woman was commenting on a painting of an eagle another character drew using the quote from a poem called "Drawing an Eagle" by Du Fu. The first two lines of the poem are a description of the eagle drawing which is what she quotes, but the poem as a whole is a commentary on the poet’s views of ambition and loathing of mediocracy. So this line in the poem emphasizes the painter’s character and values as it foreshadows his lofty ambitions which come later in the show.
Example 3:
PUNS. Puns are a huge part of jokes made in these shows. There was a show where a character’s nickname 路兄 (lu xiong) which is a homonym with 露胸 (lu/lou xiong) which meant "to be bare-chested/show your bare chest" And so another character was calling him by the nickname and he responded with "pervert." The presubs had no context which may cause confusion among viewers.