For us IPAM has been helpful to see which subnets are really in use and which aren't.
We use InfoBlox for our DHCP services. This product has an IPAM function built in but it doesn't automatically scan subnets and it only knows about addressing that it handed out. My initial belief was this was the correct place to implement and use IPAM functionality as that's where the addressing services are located.
Orion IPAM has converted me because of it's automated scanning.
First, I can get a view of how many clients are really in each subnet whether the DHCP server knows of them or not.
Second, not all subnets are DHCP assigned. Due to various reasons (political mostly) not all my subnets are centrally assigned by DHCP. IPAM gives me the visibility that the lack of DHCP would otherwise take away.
Another benefit has been with address migration. I can occasionally look through the views under "Manage Subnets" and see when all clients have been migrated away from subnets I'm trying to decom.
As for spreadsheets, we only use a spreadsheet for a handfull of /24 subnets assigned to firewalls as scanning products wouldn't be able to discover those. These subnets are not dynamic so the administrative workload is minimal.
Our internal network is way too large to even think of tracking in spreadsheets. Some of our facilities require an entire /13 address space while others simply need a /16.