As usual, this article is written from my point of view and is limited to my current level of knowledge. Fight me in the comments.
Main Purpose
The main purpose of a “Bash Bar” is to provide a replaceable, bolt on crush area for the inevitability of front/rear contact between cars. The aim of this is to absorb the impact on a part that can be quickly replaced, instead of tweaking the actual body of the car. In the event of an impact, a new bash bar can simply be bolted on or the old one can be repaired if needed.
Secondary Purposes
Alternatively, bash bars have several other benefits. Increased air flow (theoretically), added jack points to the car, lowered weight, custom mounting brackets for body panels/lights, and added space in/around the bumper area.
Hobbyist vs Pro Bash Bar
Most general drifters replace the front bumper with a bash bar that mounts to the stock bumper. Many pro-am drivers and all pro drivers adhere to their rule books to cut off as much of the frame rail as possible to add a larger crash area. This gives more engine bay space and more area to crumple without transferring the impact to the main chassis. Many pro teams run 2 separate crash supports. One for the bumper, and one to replace the cut off frame rails. Doing this allows easier replacement and more crush area.
Design Considerations
Most basic bash bars are a single bar that goes in place of the OEM bumper. The design above has the potential to puncture the tire in the event of an impact. A remedy to this would be to curve in the end piece so that the tire would hit a rounded edge of the bar instead of the corner.
Frequently, drift cars run a dual bar system. Dual bar systems are useful for oil pan and frame rail protection and a rigid mounting point for bumpers. The major down side is the added weight and complexity.
Alternate thoughts
Years ago, when it was allowed, JR ran an aluminum bash bar on the front with a secondary steel frame behind it. While aluminum is more expensive, the weight savings was likely worth the effort. Formula Drift has since changed the rules so now all bash bars must be magnetic.
Another outlawed tactic was to use coilovers with weak springs to absorb the impact but spring back out to preserve the physical chassis body. While it added weight, it lowered the amount of body work and repair needed.
The Absurd?
Samuel Hubinette for 2009 attempted to roll out a rollerblade wheel design to glide along walls. It didn’t make it into practice and Formula Drift may have outlawed this before its debut.