This is one of the many optional features in WordPress.There is no real benefit in using any of these 2 structures, but here is some details for you:
Organization:
When you are building a website which will have a lot of uploads, it is always a good option to go for the year / month structure. You might end up with 100,000 uploads in a single folder if you don’t.
As you can imagine, sorting, reading and listing these files will get a long time. There was a time when old file systems used to support limited files in a single folder ( They still do now, but it’s way more than before), so you had to make more folders to be able to save the images.
A website with 50K images and 5 thumbnail sizes will end up having 300k images in a single folder!
SEO benefits
There is no actual benefit of having a year / month structure on your files when it comes to SEO. Search engines have other tools to understand your contents, such as Structured data or HTML5 structure, which let’s the search engine understand different parts of a page without using a MicroData structure.
Custom templates – Security
Some users (such as me) prefer to have their website deeply customized, including default folder names. I myself have changed the name all wp-content, wp-admin, uploads and wp-includes folder. Some directly, some through rewriting.
So the first moment a visitor visits the website, it’s not easy to understand whether this website is powered by WordPress or not. It also makes it a bit more difficult for a newbie hacker to determine the CMS of the website.
However, with the year / month structure turned to ON, it needs one second to realize that this website is using WordPress to power up.
Although there are several ways to go around this, but still, some do prefer it.
There can be many other things added to this list, their all optional, such as the above list.
I hope this helps you decide what to choose about this.