There can be many reasons for changing the permalink structure. If you are using the default permalink structure of WordPress, then you need to change it. Default structure adds Post ID to your URLs. This structure does not provide any useful information about your content and is useless for SEO.
As I already said, its default URL Structure is not SEO friendly. And when you publish content with this URL structure, it is very difficult to find out what your content is about by looking at the post and page URLs.
Even when Google crawls your content, it crawls your post URL, to find out on which topic your content is?
Apart from this, there are many new bloggers who do not know about the permalink structure and after launching their site they start publishing articles without changing the permalink structure and later they need to change it.