关于Vue3中Plugin的类型定义疑惑？

Question

Vue3的Plugin类型定义：

declare type Plugin_2 = (PluginInstallFunction & {
    install?: PluginInstallFunction;
}) | {
    install: PluginInstallFunction;
};
export { Plugin_2 as Plugin }

declare type PluginInstallFunction = (app: App, ...options: any[]) => any;

这里用交叉类型的目的是什么？

PluginInstallFunction & {
    install?: PluginInstallFunction;
}

为何不直接写成这种？

declare type Plugin_2 = PluginInstallFunction | {
    install: PluginInstallFunction;
};