Assuming Apple continues its previous practices, once macOS 10.12 Sierra comes out, it will become impossible to download 10.11 El Capitan for the first time. That could be awkward if you want to upgrade an older Mac running 10.6 Snow Leopard at any point after Sierra ships, since you may not be able to get El Capitan then. To ensure that you can snag a copy of El Capitan in the future, open the App Store app on your Mac, type El Capitan in the Search field, and click the Get button for OS X El Capitan (it’s about 6 GB in size). After it downloads (to your Applications folder), if the installer launches and you don’t want to install El Capitan right away, choose Install OS X > Quit Install OS X to quit it. What’s important is that El Capitan is now registered to your Mac App Store account, and you can get it again from the App Store’s Purchased screen at any time on any of your Macs.