Standard conventions support the 42-inch bar height. Most premade back splashes are 4 to 6-inches tall. When that is added to the standard counter top height it allows the back splash to slip underneath the overhang of the finished bar top for a smooth transition.
42-inches is the optimal and most ergonomic height for a top that services both the seated and standing user. With a bar stool of the proper height it is comfortable to sit at a 42-inch bar. It is also a comfortable height for most people to lean against. Additionally the bar top is in easy reach of a standing person so they can set their food and drink down without leaning over.
A bar top is typically to high to serve as a good work surface and is mainly useful for leisure activities.
The main benefits for a bar top come from one person working standing up on one side and another sitting on the other. The bartender has a comfortable height to prepare drinks on his feet and you have a comfortable height to drink them while on a seat or standing.
The other benefit of a bar top is it's aesthetic appeal. A kitchen bar top can divide the space up nicely by serving as a transition area.