What is the purpose of a buffer car?

For collision protection.Also to keep a shiftable load(like a ribbon rail train) from hitting the engine. And to provide separation between a hazardous load and the engine.

This "side of the pond" they are called "barrier wagons", usually one or two empty wagons (cars) of any type marshalled at either end of potentially dangerous loads like fuel oils, chemical, inflammable/poisonous gases and substances, nuclear fuel flasks and so on. They are there to protect the load from damage (and consequent hazard) in the event of minor or moderate collision.

The attached pic shows a nuclear fuel train in the UK in the 1990s with empty hopper wagons on either side of the flask carrier, and brake van (caboose) at the rear

The only thing i can think of is to help with the sloshing of the liquid in the tanks from back to front .

The only unit trains I worked were coal trains .

One reason is to keep the tank cars and the locomotive away from each other, so that if the locomotive catches fire, the fire is somewhat less likely to spread to the tank cars and make them blow up.

They can act as a gap in the case of diesel locomotives/fuel tanks... also, in the case of moving odd rolling stock around, may have different couplings at each end, to enable "foreign" locomotives to haul it.

For the driver to take a crap in