Why are there two versions?: Web and Serialized

Sometimes authors post a series on their pixiv or Twitter, and it gets popular. Then some publisher says "hey how about you work for us, we will serialize your series on one of our platforms and even print out physical volumes for you"

After this happens usually the series restarts, and has better art and sometimes a slightly different set up, but unlike the original web version it will end up in store shelves and they are also being paid. It is up to the mangaka if they will continue the web version or not, some do not like Ore ga Watashi ni Naru Made for example, though some continue the web version extremely slowly or just for a few more chapters after serialization.

/r/manga Thread