New VLAN setup

Hey guys,

Just curious on your thoughts and advice, basically I'm being forced into some changes at my workplace. I have a windows based network ( windows server 2008-2016 ) ( Windows 7-10 for clients.) We use AD and file sharing primarily, DNS/DHCP on server side. We have 2 managed switches and about 100 clients and Watchguard firewall currently, moving to Fortigate and some other new resources for the network.

The new equipment is bringing in some mandated changes to my network IP scheme is being redone and 5 VLAN's are being instated.

My question is that based on my setup ( 2 DC's, NAS for backups, few other app server resources in my server room) that have to be accessed by all the other VLAN's for login/authentication/backups/applications. Does it even make sense to VLAN my groups of traffic away from the servers? I understand that trunking will take place and that the traffic can be forwarded, but I'm not understanding to what extent that traffic will be forwarded and if everything will need access to these servers anyways ( with the exception of like Public wifi .) What's the point here guy's? How's this efficient or secure really?

TL:DR ( Noob to Vlan's, I need a synopsis on the scope of Trunking/Tagging and why if server's are being accessed through said trunks that they are even really that much more secure)

Thanks