Full Face protection
A full helmet provides protection not just to the head, but to the eyes and face as well.
Per Sasha's answer, NBC (Nuclear, Biological, Chemical) protection requires a completely sealed suit, including enclosed head gear.
Sandstorms, underwater, vacuum, etc.
At least one of the images you show are from near future style games/movies. Integrated comms, heads up displays, and low-light/enhanced optics make such a device useful for obvious reasons.
Intimidation, anonymity, uniformity, and so on, all have a value in some situations.
In general, the helmet, due to general bulk and inconvenience (can't scratch an itch, stuffy, etc) probably won't be worn at all times, likely only being donned just prior to action, similar to current military helmets. And for scenarios where they aren't needed, lighter headgear might be provided. But there will certainly be scenarios where the fully enclosed helmet is useful, if not critical.
Additionally, people are wired to pick out faces. A shiny helmet might not help, but something matte or camouflaged could make it slightly harder to pick out targets, giving a fraction of a second or longer delay that the helmeted soldier could exploit.
And, hey, it looks bad-ass.