Samsung Galaxy S6 developer options are mainly for developers. Developers can test their apps in Galaxy S6 under various simulated scenarios, for example, how the app performs under low memory situation.
Galaxy S6 developer options also provide some useful features for advanced Android users. For example, you may want to run some ADB commands on Galaxy S6, or take a lot of screenshots on Galaxy S6 (if you just take a few screenshots on Galaxy S6 directly, please follow this guide.), or control and animation speed, or record the Galaxy S6 screen.
Please note, Samsung Galaxy S6 developer options are identical to those of Galaxy S6 edge. So this guide also applies to Samsung Galaxy S6 edge developer options.