At least on iPhone, it basically doesn't do anything.
Apple aggressively kills all background apps automatically. the "open" apps in the switcher are almost always just screenshots of previously-open-but-now-closed apps. if it was recent, the latest app or two in the switcher might actually be open, but rarely more than that.
Despite this, many of my relatives have somehow learned this habit of opening the switcher and closing all apps when they are done with them.
At least on iPhone, it basically doesn't do anything. Apple aggressively kills all background apps automatically. the "open" apps in the switcher are almost always just screenshots of previously-open-but-now-closed apps. if it was recent, the latest app or two in the switcher might actually be open, but rarely more than that.
Despite this, many of my relatives have somehow learned this habit of opening the switcher and closing all apps when they are done with them.