I know a person who bought several hundred domains redirecting to his main site. Does it really help as per SEO or marketing anyway?

Does it help?
From what I have seen, it really doesn't help much. That person I mentioned above gets less then 10 hits from those domain redirects; whereas he have to spend several hundred dollars maintaining those domains. Unless they are premium domains or highly searched, buying tons of domains does not really help in marketing.

Best practice
If you happen to have decided to have many domains whats the best practice? Best would not be redirecting to your parent site. Best approach is having separate websites for each of them. Make sure they have different contents with links to your parent site. As you link to the parent site put your target keywords as link anchor text. If you do that, your domains will get indexed on search engines for different keywords. Also gets your parent site backlinks.