DHCP relay is one of those things that isn’t nonsense but if you need it you need it… otherwise do not enable it.
The way DHCP works is basically the client sends an ethernet broadcast to find the DHCP server.
DHCP Broadcasts (well, all ethernet broacasts) do not normally cross VLANs (as a VLAN is a single broadcast domain), and as a result you need to have a DHCP server on every VLAN (or multi-home your DHCP server which is bad and joining networks together - defeating much of the purpose of running multiple VLANs for security isolation).
This is where DHCP relay comes in. The switch will forward DHCP packets to a machine in the other VLAN where your DHCP server is.
It’s to reduce management overhead. If you had say 10 VLANs with DHCP required, you’d need to manage 10 DHCP servers. Pay 10 OS licenses, etc. Your information will be scattered across 10 servers. You’d need to have 10 servers with permission to update DNS in active directory, etc.
FAR easier to just use DHCP helper on the switch.
If you aren’t doing any of that, leave it turned OFF. As per security best practice.
You will not reduce broadcast traffic by enabling DHCP relay. The switch doesn’t even see the traffic to relay until the broadcast is sent by the client. The replies aren’t broadcast - they are directed back to the source MAC.
Broadcasts will still be sent by the clients. All this does is forward them across VLANs to a server AFTER the broadcast is sent by the client and already on that VLAN.