I found a way around this, not having to use it, but still would like to know why it's there. Thanks!
The button "Assign IPs to Existing Subnets" do exactly what the name is: Assigning the orphaned IP addresses into existing subnets in IPAM; if they fits into subnet's range and IP addresses do not already exist in subnet.
While IPAM works correctly (no crash or power outage during Add/Remove subnet and it's IP addresses) this button will be always grayed.
We work hard, so you never need to use this button :)