The answer of this question is really an ideological dilemma, every user has his own reasons, but among the most common causes,
* place a CV or a Portfolio online.
* Place an online photo gallery.
* Use the web as an Electronic wide spread Business card.
* launch a personal website (singers, artists, Djs', Advocates, and other professionals).
* launch an online community.
* launch a News management and delivery system.
* launch an online front for a Shop or another business.