Does it need to? No, like you said, there's plenty of comics that just present themselves with a normal comic-page style layout who've found a good-sized audience here. Would those comics do better if they were in the vertical Tapastic-style format?
m-maybe???????
The vertical scroll-y format is designed to work well on phones and mobile devices, which, since Tapas is a mobile app, is a lot of the audience on here. The page format works on phones and mobile devices, but wasn't designed for it and isn't always ideal. So the idea is that you might reach the Tapas audience more easily if your comic is laid out in a way that looks good on the devices that the majority of its readership is using.
I've seen one or two creators say they felt it was worth it to reformat and that doing so gave them access to a wider audience, but ultimately I'm not sure if there's any hard data one way or another.