The Knightcrawler was probably constructed during World War II by Wayne Technologies under the authorization of Thomas Wayne to get military contracts from the military. But the best minds of Wayne Technologies couldn't make it run and as a result it turned up in Wayne Aerospace Hangar under the ownership of Bruce Wayne.
After realizing the impending doom of the New God Stepppenwolf, Bruce Wayne works on the Knightcrawler and gets it running as an additional help for the incoming battle with Steppenwolf.
It is a four-treaded tank with each tread on a separate leg so it can walk. It can pierce into surfaces with each leg to climb up walls or hang from a roof. Its cockpit interior is based on function, like a forklift. It has one command seat and two fold-out benches. 
The Knightcrawler also has powerful weaponry for dealing with alien threats, as seen when Batman used the vehicle's Gatling guns to tear through a Parademon's armour and flesh. This addition of such weapons was likely added in order for Batman to stand a chance against the Parademon threat facing the Earth.