Hi there,
As I understand it, you can benefit from redirecting related domains to your main site in a few ways:
1. Purchasing domains that are similar to your own can be a tactical decision to help reduce competition.
If you own them, no one else can use them. Also, if these 'similar domains' possess keywords your main site ranks for, it will be harder for copycat businesses to register these domains, pop up and compete with you directly at the domain level.
2. There is SEO value if you are purchasing the domain from an established website that has established relevant content.
As I understand it, simply purchasing an unused domain that's similar to your own won't have any inherent SEO value (except for country specific domains which I touch on below). A primary factor in determining how much 'google juice' google allocates a domain in the search results is content. So in order to really benefit from additional domains, you would need to flesh out relevant content on these domains and link through to your main site. Ideally these domains would then rank alongside your main domain and push your competitors down a few spots in the search ranking. Your main site would then benefit from the network of links from the 'outlier' sites which feed back to your main site. The major con here is the time and effort involved.
Alternately, you could keep your eyes peeled and when the opportunity arises, acquire a similar domain from an established competitor who is trying to sell up. In the latter case, (if his DA is high enough and content is relevant) by creating redirects from his pages to your main domain, google will pass his 'google juice' to your main domain, and that's beneficial. The major limitation here is that it's not often you can buy out a competitor, and when you can, they often have a skewed view of how valuable their domain actually is.
3. Purchasing similar domains that are country specific could be beneficial.
If you have an international audience it can be beneficial to purchase country specific domains and link these back to your main site. For example, you main sites domain is http://www.theatermania.com/. Purchasing the .co.uk version would be a good idea if you wanted to help customers in the UK find you in the search engines when they search from the UK. As far as i'm aware, more weight is lended to a .co.uk domain when searching from the UK, so this domain may appear higher in the search results than your .com when appropriate phrases are entered into the engines.
This is my understanding of it all! I hope it helps you in some way. And to the other readers, correct me on any point if I'm wrong / slightly off, missed anything.
Jake