Origins of Scanlations
In the beginning, scanlations were started as fan projects because there were few commercial releases in countries outside of Japan, and importing manga was difficult. Friends living in Japan would scanlate a manga for a friend. What may have started out as a favor became a project, and grew from there. Eventually well-organized manga pirate groups were formed, which usually had their own webpage as well as an IRC channel. The latter was arguably more important in shaping the nascent scanlation community because IRC allowed real-time interaction between the group staff and the target audience. IRC allowed the groups to easily recruit new staff and also provided a means to distribute the scanlations via fileservers. These fileservers, which could be created by anyone in the IRC channel, helped reduce the burden of distributing the manga scanlations. This act has been recently called scanlation piracy.
The scanlation scene is younger than the anime fansub one; most of the older organized groups were founded after the year 2000, although the magazine Mangajin, which used (legal) manga extracts and translations to introduce contemporary Japanese language and culture to western youth, was first published in 1990. Recent years have been marked by an explosive increase in the number of groups and consequently, scanlated manga available. Several sites list daily releases and maintain databases on projects and scanlation groups. These sites can be used for tracking releases, searching by genre, looking for synopses, and so on.
A few people today probably do scanlations simply because they want to see their favorite manga unedited (no missing scenes, flipped panels, etc.), which has occurred especially for manga that is aimed at a younger audience, such as the popular Dragon Ball series. The Dragon Ball portion of the manga released in North America is now released uncut due to fan demand.
Recently, most new anime is fansubbed, and much of it is also sooner or later licensed for distribution by companies around the world. However, due to the sheer number of manga series in Japan, this will probably never be the case for manga. Scanlations are often done to translate manga to a language it would never be published in. A dramatic example being the few groups who focus on alternative and/or adult oriented manga (seinen and josei manga) that tend to be overlooked by publishers in favor of more popular shojo, shonen, or manga aimed at anime otaku.