From what I understand is that IPAM can read the ARP table (or more precisely the ARP cache) which should have the MAC address and the corresponding IP address of every device / endpoint that has sent data through this router.
Since ARP cache is unreliable and can even be totally empty, the use of Neighbor Scans is disabled by default.
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Adding on to this,
The main purpose I find for neighbor scanning is when you have nodes that do not respond to ping your gateway router will still know that they are out there so checking with the router is a good way to double check that open IP's really are open. The trouble is that hitting a router and asking it about all the subnets it has interfaces on can be taxing on the router, less so with new hardware but 5-10 years ago it was pretty easy to lock up a router with just SNMP requests asking it what all it was set up to do.
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