IP Address Management (IPAM) tracks which IP addresses are assigned to what. Without IPAM, you'll eventually assign the same IP twice and cause an outage.
IPAM answers: Is in use? Which IPs are free in the server subnet? Who allocated this IP and when? What DNS name maps to this address?
Small networks survive with spreadsheets. But spreadsheets don't detect conflicts, don't integrate with DHCP, and get stale fast. When you grow past addresses, you need proper IPAM software.
IPAM also tracks subnet allocations, recording their purpose, VLAN assignment, and remaining space.