Why should you submit a Google Sitemap?

To show your blog post on Google ❤️... Very simple answer... Other wise you will need manually indexing every post
. Which is very tough

A sitemap is a basically a file with links to all the pages and posts of a website.
- pages
- A
- B
- posts
- category A
- category B

so, this file helps crawlers to search all site programmatically.
And as search engines, do use crawlers, submitting a sitemap helps to crawl known areas faster than by searching itself( chances are google may don't know your post until yu have a good traffic)
So, a sitemap helps google in faster indexing and searching.

Just imagine that you have a 1000 page book but it does not have an index. So how long will it take you to find each chapter? And where is the chapter? How to know? Is it or not? Exactly the same happens with the spiders (bots) of a search engine, when they crawl a blog or website without a sitemap. That is, Spiders also take a lot of time to find each page and understand its location. That is why Sitemap is very important. It helps Search Engines a lot in crawling a blog or website.

Basically the sitemap acts as a map for Search Engines. Due to which it is easy for Search Engines to understand the structure of the site. That is, which page is where? Which Category or Sub-Category is Under Listed? And what is its path? Search Engines do not take any time to understand all this. With this, search engines are able to easily index all your posts, pages, images, videos and URLs etc. That is why Sitemap is very important for a blog or website.