The program creating the traffic controls the mark initially. Then at any hop along the way it could change. So on ingress it would be rare for DSCP to be meaningful. Your LAN is under your own control so it makes the most sense to mark traffic to your liking at ingress. Unfortunately the iptables rules for ingress don't get run until after the ingress queue is already processed. (nftables solves this problem but isn't compatible with the default firewall on OpenWrt).
On egress to WAN on the other hand, you have some control of all the hops from the device that produces the packet through to your router. Some games are marking packets already, but you can also remark them during routing.
If you're a gamer, and you're still not happy with your QoS, marking packets is your best bet, but it requires a bit of a deep dive... certainly deeper than filling in some boxes on LuCI. Here's the current mega-thread on developing marked packet strategies: Ultimate SQM settings: Layer_cake + DSCP marks