Hackers typically:
steal financial information (credit cards, bank account #s, etc.) and/or PII (SSN, DLN, DoB, names, etc.) steal usernames/passwords then use those to attack further install malware that captures keystrokes, screen captures, etc. install backdoors and/or botnet software on victim's machines so they can attack other computers around the world exfiltrate private and sensitive data, emails, etc. modify data, emails etc. residing on the hacked network capture network traffic and so much more
Not only can the technical damage be detrimental to an organization, but a damaged reputation and the expenditures to fix the damage after an attack can also be catastrophic.