This is Mego's 5 3/4 inch Diecast Metal Batman figure from 1979. Mego actually produced 4 characters in this series with the others being Superman, Hulk, and Spiderman.
I'm not sure where Mego was going with this series. The scale was odd for the time since the other Mego lines were being produced in 8", 12", and 3 3/4" sizes. I can't think of any other figures from this era being produced in this scale. In comparison with modern figure lines, this Batman fits somewhere in between the sizes of JLU and DCUC figures.
I can only guess that diecast metal was used on these figures as a way to increase the perceived value of them. Also, there were quite a few other toys at the time being produced with metal pieces... many of them being Japanese import type toys like the mini-Shogun Warrior figures and Mego's own Micronauts. However, your guess is as good as mine why Mego decided to go the "metal route" with these heroes.
The execution leaves a little bit to be desired as the only metal piece on the figure is Batman's torso. The rest of the figure (arms, legs, head, etc) are made of fairly traditional plastic. The joints on these figures (at least on this example) are very loose and I can only imagine that this has become worse over time as the figure has been played with more and more. So, with the heavy torso and loose joints, standing and posing this figure is a bit like trying to balance a hard-boiled egg on a couple of spaghetti noodles. The weight of the torso is just too much for his weak knees and ankles and causes this figure to topple unless everything is just right. I had to employ a bit of masking tape under his feet to help him hold many of these poses.
Stylistically, you can see that this Batman figure has the looks of his 8" older brother. The cape is the same, prone to fraying, nylon material that you would find on the 8" Megos and his head sculpt is also very close to that of the 8" Mego Batman.
From what I can tell, these Mego diecast figures are fairly rare. I haven't seen them pop up for sale all that often. I like to seek out unusual Batman figures and this one doesn't disappoint, in that regard. I picked him up, at some point, last year and he makes a great addition to the collection.