Im general you bridge clients to the LAN (as in Network 1) to allow direct connectivity between the wired and wireless devices. This allows broadcast traffic to work, etc. You would use NAT mode (Network 2) for a variety of reasons including conserving IP addresses, security (can combine with firewall settings and use for a guest ssid), or perhaps something else. It also could have just been a misunderstanding by whomever originally set things up. The understanding of why yours is that way, that you have presented doesn't make much sense to me as a reason. Suffice it to say there could be valid reasons to have your setup that way. In general you mention eliminating SSIDs, you may want to look into : https://documentation.meraki.com/MR/Encryption_and_Authentication/IPSK_Authentication_without_RADIUS This allows you to consolidate SSIDs while still providing different firewalling and vlans and other rules to clients. The caveat being you can't mix match NAT Mode/Bridge mode, all clients need to be one way.
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