There’s no benefit in registering multiple domain names just to forward them to your primary website.
Well, automatically redirecting from one website to another, especially from a home page, is generally a no-no. It’s not considered a best-practice approach. If those extra domain names do not have a history (ie, they were once used for a quality site that hosted good content), Google will simply ignore them as they have no value.
So Then What?
If you’re sitting on a bunch of parked domains and want to put them to use, go ahead and build a small 3-5 page website on them, referred to as micro-sites or mini-sites. The cost is minor, and you’ll be implementing a strategy vs attempting to score some easy hits. Keep the content on each website fresh, unique and toss in some quality pictures and/or video. You will now have a website that potential customers can find, and that website will ultimately forward leads to your primary website. It’s like having a small army of sales people (websites) online that are all forwarding leads to the mothership.
SEO aside, there’s a good reason you might want to automatically redirect a domain name to your primary website – and it pertains to advertising. Let’s say you own “Expert Audio Engineering Inc”, and your primary domain name is “ExpertAudioEngineering.com”. You may also own “WeHearYou.com” which is short, sweet, and just has a nice ring to it. You may discover that “WeHearYou.com” is more memorable and decide to use it in some of your advertising. When people type this domain name they’re automatically redirected to “ExpertAudioEngineering.com”, which is fine, but don’t expect “WeHearYou.com” to go #1 in Google – you’ll be lucky if it’s even on the radar.