Once a hacker has gained access, they want to keep that access for future exploitation and attacks.
Once the hacker owns the system they can use it as a base to launch additional attacks.
Hacker may just hack the system to show it was vulnerable or he can be so mischievous that he wants to maintain or persist the connection in the background without the knowledge of the user. This can be done using Trojans, Rootkits or other malicious files. The aim is to maintain the access to the target until he finishes the tasks he planned to accomplish in that target.