That is the mounting point for night vision goggles. The following image shows a helmet with NVG mounted.
Helmet mounted NVG; image from lemt.by
The USAF has helmets with different mounting and some of the modern pilot helmets (like BAE Striker II, for example) have an integrated night vision device.
There are several reasons:
A helmet provides head protection. A fighter airplane can make many sudden turns and a helmet provides reduction of the risk of a head injury.
If the pilot needs to eject the airplane, helmet is needed to protect from wind blast, in addition to what I mentioned #1.
Helmets play an important role in noise attenuation.
The mic and ...
In addition to ejection it's to protect the pilot's head from impacts against the inside of the cockpit in case of sudden maneuvering or loss of control. Many aerobatic pilots wear helmets for this reason as well.
The pilot's helmet in the photo is a standard Gentex HGU-55/P fitted with a Thales Aerospace Scorpion Helmet Mounted Cueing System. Information on the SHMCS is available on the corporate website
As to weight, the display, cableing, and visor add several ounces of extra weight to the helmet. It's a little heavier ...
Primarily to protect the pilot's head from windblast and and trauma from blunt object impact in the event of an ejection and parachute landing. The current generation helmets do not offer ballistic protection against enemy fire
That helmet is called a helmet-mounted display, or just HMD.
It offers a display much like the HUD in front of the pilot.
So when they look away from the HUD, vital information moves with them. Including targeting information. And even FLIR overlays.
Some models even allow the pilot to select a target by just looking at it. Like the Eurofighter.
As for ...
Those are also mounts for Helmet Mounted Sight for the Vympel R-73 missile. Both the MiG-29 and Su-27 were equipped with this missile system.
I think F-35 pilots have this technology now but with a fancier helmet. The old ones on the MIGs looks like monocle.
There was at least one case when a helmet saved a helicopter pilot's life, namely a RAF's Chinook HC4 pilot. In short, Flight Lieutenant Ian Fortune was injured while extracting some soldiers in Afganistan. An enemy bullet basically struck his helmet above the upper rim, crushing it and sending parts of the helmet and visor into his face, but nevertheless ...