Because my brother posted about mecha recently, I guess I might as well explain some of the reasons I like the genre so much. In particular, I want to talk a bit about why giant humanoid robots are such a good idea from a creative perspective (since explaining why I am such a fan of giant robots is an enigma even to me). In no particular order, here are some reasons:
1) Mecha are not real. People do not actually use humanoid vehicles for military purposes in the real world. This makes them very different from things like tanks or fighter jets, which actually do exist in the real world. This means that the player of a mecha videogame (or a person watching a mecha anime), can't bring any real world preconceptions about how mecha should behave into the experience. This has a few drawbacks, but it has the huge advantage of letting the game designers do pretty much anything they want with mecha without worrying about violating a player's suspension of disbelief.
I'll use an example of this advantage at work. Real world combat vehicles are extremely limited. Anyone could tell you that a tank fights only on land, and can't go out into the ocean, fly, or fight in space. What is more, a tank is mostly a weapon for fighting other land-based vehicles. A tank can't shoot down a fighter jet, and under anything resembling normal circumstances it won't be used to attack an aircraft carrier. A mecha, however, is not limited in such a way. Many mecha series, like the influential Mobile Suit Gundam, let mecha operate in any terrain, and fight any opponent. The same Gundam can be used to fight tanks in the desert, fly through the sky to shoot down jet aircraft, or even go into space and fight spaceships. At the same time, another mecha series might limit mecha to being simply walking tanks, fighting in the same battles and against the same opponents as tanks. Mecha are only limited by the very loose tropes of the mecha genre, rather than by assumptions rooted in real world experience.
2) Mecha are humanoid. Even though mecha are (usually) lifeless machines and weapons, they can resemble a person. This resemblance makes it easier for a game designer to add anthropomorphic touches, giving a mecha the illusion of personality and individual identity. It is hard to make a tank look like anything but a tank, but it is relatively easy to make a humanoid machine look like it is smiling, like it is wearing a mask, or make it look masculine, feminine, evil, or heroic. It is even easy to give mecha body language. All of these things allow a humanoid robot to be much more easily turned into a character in its own right than a non-humanoid machine.
3) Mecha operate like a hybrid between a vehicle and a person. It is possible for a mecha to use hand-held weapons like a human would, integrated weapons like a vehicle would, or some combination of the two. This is probably a side-effect of the above two factors more than anything else, but it allows for a lot of flexibility regarding action and gameplay, and as such is a noteworthy advantage. This allows things like melee combat between large vehicles, or equipment customization mechanics for large vehicles. It also allows a game designer to introduce tropes of human vs. human combat into mechanized combat, adding flexibility. Finally, mecha with hands can do things that non-humanoid mecha can't, such as open doors, grab things, throw things, rip things apart, etc.
I could probably go on about several more details, but none of them are as specific to mecha as these three, so I will leave them for now. Any more and I might reveal a bit too much of my rabid mecha fandom...