I do it just to make things easier on me. For example, I sub to my work on Webtoons because it makes it easier to get to it. I use the App quite a bit and by having it subbed, I can easily move my finger to "My Webtoons" and just pick it off from there. Helps that the App usually saves your spot so its usually less work there.
Otherwise, I'd have to go up and type out the series name, click on it, etc.
Ultimately, it just makes things easier and since Webtoons hasn't learned what "notification" is, the quicker I can check on my work, the better.