Surprised this question doesn't have any answers yet.
There are multiple use cases. I'll point out 3 different ones.
1. It's supposed to act as a buffer. It lowers risk of screwing over your domain by spamming directly but still sends a high percentage of link juice - less than that of a PBN or guest post.
2. You can also change 301s to different locations. So if you are churning and burning you can switch domains quickly.
3. You can hack DR/DA metrics pretty safely also using buffers.
Redirect backlinks take advantage of a Google mechanic that allows you to switch to another domain while still maintaining the link juice from your old domain. 301 Redirects allow the new domain to gain, as well as traffic, from the links pointing to the old domain. You buy a domain that has a significant portfolio of relevant links and 301 redirect it to your current website, thereby benefiting from the link juice.