To answer the question of "What do you gain?".....
You will or hope to gain what you set out to do before you even turn a bolt to disassemble the engine, start with a plan of what your goals are, many people rebuild for more power, changing parts, grinding this or that, etc, to gain HP, engines are also rebuilt for economy, Can also be called "freshening" to make the engine like new again. Others, like me, rebuild because of enjoyment and making parts work together that were never inteded to be in the same engine, looking for that "extra" edge for performance and/or economy.