Covers are extremely important. When I was first starting out on RR, covers targeting my target demographic easily doubled my follower counts within a week. The vast majority of people
do judge a book by its cover regardless of what they might say about not. It's just how us humans are. They make your story look more professional, so people will assume it's higher quality. Not to mention that depending on the sort of cover you get (such as one with attractive women on the front of it), it doubles as clickbait. As much as people like to say they hate clickbait, it works very, very well. A good cover can make or break a story, especially on platforms (such as Amazon) that have a large number of stories. I have several patrons on Patreon and even more free readers who have all admitted that they had no interest in my stories until I replaced their free use covers from Google with proper covers.
I opened a poll in my first chapter for the reason they decided to read my story. (Only 9 voters so take the result with a grain of salt) The most votes have gone to Genre (6), followed by Title (4), then Synopsis (3) and Cover (2). Baiting through Rec or Ranking got no vote.
This makes sense for what caused them to
read the story, but it's the cover and title that gets people to actually
click on your story to learn what it is about in the first place. In that case, a picture is always going to be more eyecatching than text. You have to grab their attention and then convince them with the rest to actually read the story.
Well. I would wait until i got 50k readers on webnovel or royalroad. But having your own cover is great. You won't have to worry about being sued if you later publish it
50k readers is 5 times more than what the highest-followed fictions on Royal Road have = P. Webnovel originals don't get that many follows, either. If you have even 2k followers, you have enough to potentially make a living off of Patreon with proper monetization, and a cover will help you get that many followers so long as you have a story interesting enough to keep their attention after they check it out because of the cover.
Now, to answer the original question as to whether it is worth it or not... it depends entirely on your goal. If your goal is to get as much of a following as you can, yes, they are worth it. If your goal is to eventually self-publish your story, yes, covers are even more worth it. If your goal is to just post on the internet for fun and not worry about all that other stuff, then it's only worth it if you want a cover just because.