One of the symptoms of a mature community is the appearence of trolls. A troll, for those who are not too acquainted with Internet language, following the Wikipedia “is someone who posts inflammatory,extraneous, or off-topic messages in an online community, such as a forum, chat room, or blog, with the primary intent of provoking readers into an emotional responseor of otherwise disrupting normal on-topic discussion”.
In conclussion, somebody who is bored and spends its time annoying other people on the Internet.
But it´s also true that if the troll is under control, it can be very useful and helpful for the community in the following ways:
Increase the engagement of the community
Trolls are a good to measure the engagement. A community with a strong engagment will defend their virtual territory, the community will put the limits and will try to scare the troll.
Create and hallmark for the community
Some of the messages left by the troll or some answers can became into a hallmark for the community, as memes, animated gifs… and, in consecuence, something to use in our profit.
The more troll appears the more our community will interact
Usually trolls are quite active users, and when the other members of the community start to interact with them they multiply their activity. This fact will create more content. Probably, part of this content will have a high quality. That´s why the community will try to fight the troll using rational and logical arguments.