Why does my computer screen turn black by itself even when I was connected to AC power ?

Even when I was connected to AC power with the power indicator on the left of the unit on, why does my computer screen turn black by itself ?

[To reproduce the symptom]
1) Turn on the computer, keeping the AC adaptor connected to it.
2) Open [AC Profile] of PowerPanel, check the [Standby Timer] of [LCD/Video] and leave it as it is for 25 minutes (The initial value is a 25 min.)

Due to the initial settings of PowerPanel, the power saving function is in operation. Pressing any key on the keyboard or touching the touchpad recovers the screen.

To disable the power saving setting, perform the steps below:
1) Right-click at the "Power Panel" icon on the taskbar and select [Edit/Create Profiles].
2) In the [Profile Editor] window, select [AC Power] and double-click at the "LCD(Video)" icon.
3) Right-click at [Standby Timer] and set it to [Off].

[Note]:
The standby timer is a power saving function to cut the display output when no operation is executed on the computer for a certain period of time.