Creating alternate ways to regain access to a host without going through the exploitation phase all over again.
Re-exploitation isn’t always possible: Some unstable exploits might kill the vulnerable process during exploitation, getting you a single shot at some of them.
Gaining a foothold is hard to reproduce: For example, if you used a phishing campaign to get your first access, repeating it to regain access to a host is simply too much work. Your second campaign might also not be as effective, leaving you with no access to the network.
The blue team is after you: Any vulnerability used to gain your first access might be patched if your actions get detected. You are in a race against the clock!