If you replace NOOBS with PINN (it's a modified version of NOOBS with improvements) and connect your Pi to the internet, you will have access to 30-50 OSes that you can download, install and try out. Depending on the size of your SD card, you could install multiple OSes to your SD card and decide which one to run when you boot it up.
If you have only Raspbian installed, it will boot into Raspbian if you do nothing at startup.
If you have multiple OSes installed, you will get a menu to select which one you want to use. if you do nothing, it will select the last used OS for you.
If you press the Shift key on startup, as directed when the Raspberry Logo appears, Then you can get into the NOOBS or PINN recovery program whcih will allow you to download install and try out a different set of OSes.
Additionally, PINN will allow you to replace each OS individually, whereas NOOBS requires the whole SD card to be wiped and reinstalled.
(See my signature for a link to the main PINN thread on this Forum, which will take you to the documentation pages on github and the pinn download from sourceforge. You can install it like you do NOOBS, or there is a version that can be installed by Etcher).
The official documentation and quick start guide is at
https://github.com/procount/pinn/blob/master/README.md and
https://github.com/procount/pinn/blob/m ... ME_PINN.md
(DarkPlatinum just beat me to it!)