If history is any indication, when macOS Sierra 10.12 is released in late September, OS X El Capitan will disappear from the Mac App Store. If you “get” it (it’s free), it will be in your purchase history for download again.
You don’t even have to install it. But if you don’t click “Get” now, it won’t be an option in the future when you discover that you need to upgrade your software to maintain security and compatibility.
You can get El Capitan from the Mac App Store here. After it downloads the installer will run, which you can simply quit. At a later date if you need to upgrade you can run “Install OS X El Capitan” from your Applications folder. Or you can return to your “Purchased” tab on the Mac App Store to download again.
Even if you have no intention of installing El Capitan you really should add it to your account in case you need it in the future. Especially if your computer is not compatible with the upcoming macOS Sierra. Why? Apple traditionally releases security updates for the current generation of software plus only two previous generations. Meaning Mavericks stops getting security fixes next month and Yosemite stops in September 2017. El Capitan gets security updates until September 2018. When Apple stops releasing updates then developers often stop updating and testing their software for that platform. Sometimes you get forced into updating in order to keep using the software you need.
In days gone by, our computers were stand-alone tools. You wrote documents, crunched numbers, or played games. It didn’t matter if it had 10 year old software so long as it did the task it was programmed to do. Security wasn’t a concern. Now computers have an entirely different purpose. They are communication products, service nodes on a much larger computer. In order to cooperate and be good citizens of the internet and to remain relevant amongst their peers they need frequent updating. And, sadly, at some point maintaining software on older machines for a small number of people loses out to the majority of customers and potential customers who want the money spent on new features and fixes. I’m not saying that this is or isn’t the right way, but at the end of the day these are free market corporations, not charities, and they have a responsibility to their shareholders make a product and service that their customers want.