It will act as a late game unit that can swing the war in your favor it it isn't challenged......
Of course, in the game, you can turn everything into very powerful stuff.. Even it's just a vehicle which made by papers.
In the game, it's just a texture with an image.
I mean, in reality...why should someone invent this stuff. In most games with mechs. Those machines were invented earlier and replaced wheel vehicles. So, it's fine. However, this concept of art is not just a common mech. In the other concept art, people can make the flying battleship. So I think a flying air carrier will be much more reasonable? Instead of a slow movement carrier fortress.
About the cavalry, I have the same question too. New cavalry become the medieval heavy cavalry for what? And their rough lance also can't compare with Knight's metal lance(because easy to broken). Cavalry should kill unarmored units much easier, also with speed. If you want to let cavalry put heavy armor on. Why don't just let mech do their job?
The whole new unit images are blend with Steampunk(that recon mech), medieval style and anti-gravity elements.
About the reason why those nations (I don't know) choose to develop these kinds of stuff, it's still a riddle.