Multiple domains for the same website, what do you think?

 I saw many times the situation when several domains are used for the same website. A common sense can be easily understood when for example different domains are used to demonstrate website pages are presented in several languages! But personally I don?t understand for which purposes the same website is made available under different domains in other cases? What advantages can be taken from displaying a website under different domains?

if the domains are relevant to the sites cotents then i cant see why there would b a bad side to this.

These all have been done for SEO purposes.

to redirect a user to your site if made a mistake with the extension used ? to prevent the competition use the domain ?

How can multiple domains help in terms of SEO? As it is known many search engines don't like domain redirects!

It could help in SEO only if the content is different to each site. At some of my clients where their products are not related to each other, we create one site per product. This way you can target specific terms for each product while in one site that would be very difficult.

maybe it's just for name variation.
CMIIW, i am newbie in SEO.

Exactly:  A good example is setting up a commerce site for a client who needs their products to "shine" so it would attract more people who visit their specific site.  By setting up either 1 or a group of similar products on individual pages using a different domain rather than setting it all up on the same domain, they are all bound by the same terms rather than specific ones.  In that aspect, it's not as "web 2.0" compliant and would yield the company in question a lot less visitors and inevitably less profit margin.  Talk about completely anti-SEO in that aspect...no one wants to drop in their rankings.