The display is typically the most power-hungry component in any phone because of the backlight. By removing it, the iPhone will be more power efficient, which is great for users.
It’s not the only reason to applaud OLED. Getting rid of the backlight allows for the entire display module to be thinner, which is an important consideration in a smartphone. Apple could use the extra space to make the phone thinner or add a little more battery capacity.
Just as important is the image. OLEDs display more vibrant colors, have deeper blacks and brighter whites and a greater contrast ratio so most people find them superior to LCD.