A common misconception is that registering multiple domain names and pointing them to the one website will help your seo, this is simply not the case. They add no advantage simply redirecting to the one website. It is better to have one main website and domain name and focus on promoting it.
The only way to appropriately re-direct domain names to the one website without it actually harming seo is to set up 301 Permanent Redirects that point to the main domain name.
Established sites / domain names
Buying the domain name of an already established website and then redirecting it to your own however does have seo significance, as you are gaining the value of their aged links etc, this is an ideal situation when a business it bought out by another company, and they use the old businesses domain name to redirect to the new company.
Country Specific Domain Names
If your business is on an international level then there is a lot of value in registering country specific domain names. I.e if you are doing business in England and you have a.com.au domain name you will have a hard time ranking at all in a country specific search without a .co.uk domain name.
Securing the Name / Brand Protection
If you have a business in a competitive area, or a great business name / idea, then registering multiple domain names simply so others can use them is a great idea.
For example, our main domain name is www.spgraphics.com.au purely because its short and sweet, however we also have www.showponygraphics.com, showponydesigns.com.au, showponygraphics.com.au etc purely to stop others from registering them, and trying to capitalise on our reputation and good name which actually happens more than you might think!
You might also wonder why if you search for a specific keyword, that the website with that keyword in the domain name doesn’t come up first? That is because search engines do not rank websites on one singular method, they actually rank websites according to a wide combination of reasons. Having good relevant content is actually better than just having your keywords in your domain name with no relevant content.