What does “scope” do in ip route and why it is necessary to setup static route in Linux?

If I want to replace default dhcp route rules with static ones, I have to add a rule ip route add scope link dev eth0. Or I will get an error: “Nexthop has a invalid gateway”.

Here are my questions:
Q1: What does “scope link” mean in up route?
Q2: Why it is necessary to change from dhcp rules to static rules?