Rendering standard HTML pages is a simple two-step process: crawl and index. Search engine bots request a page from your website, scan the text and links, then index it. At that point, your page can be ranked among others in order of relevancy for any search query.
Google is on record as saying that it can crawl JavaScript content. While that is technically true, most SEOs agree that JavaScript adds a level of complication you must simplify in order to get your Angular site indexed efficiently.
This is because JavaScript pages require Googlebot to render the content first before crawling and indexing can happen. That added step takes more time and resources, and it often means Googlebot will need to visit a page multiple times before it indexes all the content. If Google is one of your main sources of traffic, you just can’t afford to wait.