How expired domains help in SEO?

They may help if ....

They were not dropped because of a penalty.
They may help if they have a good backlink profile.
( which is the primary reason to acquire them )
So if you vet the domain for past backlinks and penalties and it passes with flying colors, grab it quick then.

Then you use 301 redirects to your money pages, notice I didnt say site.

What you do is examine the backlinks that currently exist and you include individual 301 redirects to the appropriate pages on your site. All the rest you 301 to your home page.

Once domains expire, they do not lose all their authority. In fact, SEO consultants can attest to the fact that these expired domains are sometimes favorable for SEO. This is why several websites hold auctions and bids for expired domains for others to buy.