What's the difference in defining "repositories" in build.gradle vs settings.gradle?

In many of our projects we have the repositories block defined in both build.gradle and settings.gradle, they look like this:

build.gradle

repositories {
    maven {
        url { custom_nexus_repository }
    }
    maven {
        url { custom_repository }
    }
}

settings.gradle

pluginManagement {
    repositories {
        maven {
            url "${custom_repository}"
            allowInsecureProtocol = true
        }
        maven {
            url "${custom_nexus_repository}"
            allowInsecureProtocol = true
        }
    }
}

What's the purpose of defining it in both settings.gradle AND build.gradle? Wouldn't one or the other be sufficient?