If you do not optimize your permalink your visitors and readers may get confused with your published post while searching on search engines.
i.e http://www.ansmachine.net/2015/10/how-to-install-wordpress-locally-on.htmlFor example you can see above link, that is not complete and optimized you have put best and search engine friendly and keyword included permalink to optimize your post. To get higher visitors, to increase rank and to increase organic visitors from search engine.
The above link is saying how to install WordPress locally on.................., It is not complete where to install WordPress, there is only one keyword "WordPress" but it is not enough so you must have to put better and optimized permalink for your post.
i.e http://www.ansmachine.net/2015/10/how-to-install-wordpress-locally-on-windows10-with-xampp.htmlThe example of optimized and perfect permalink is mentioned above, you can see now it has keywords when user searches on search engine the search engine will search according to the permalink keywords and title keywords...
To edit permalink after publishing the post,
It is not that much tough to edit permalink, you just have to put your published post on draft by clicking on revert to draft, that can help you to edit your permalink again on blogger.
Yeah you can edit your permalink by clicking on revert to draft but it has few disadvantages, if that post is published long time ago than after editing the post that post will display 404 for error to your visitors.
You may lose your loyal visitors, those visitors who are seeking for that particular post they may get frustrated with your changing permalink.
So that you must have to redirect your previous post to newer post... that will be better to gain profit without losing anything...