Quite simple. Lets say you have a SaaS or s.th. and a client won’t want to index it on Google. But another client wants. Its a whitelabelservice or s.th. similar. So based on a condition you integrate the Meta Service from @angular/platform-browser and use it to update the meta. Se following. Main Code is:
this.metaTagService.addTag({ name: 'robots', content: 'index, follow' });
this.metaTagService.addTag({ name: 'description', content: metaDescription });