Question

Why is a just-in-time compiler useful for executing Java programs?

Answer 1

JVM interprets the bytecode instructions one at a time. 
Usually interpreted environments are slower than running native binaries(C, C++), because interpretation process requires converting each instruction into native machine code, everytime the program runs.

so, Java uses a JIT(just-in-time) compiler to make this interpretation process faster)
A just-in-time (JIT) compiler compiles the bytecode for a method into native machine code the first time the method is encountered.
This means that the Java program is essentially running as a native application. 
 
so, A Java program that is run by a JIT rather than a traditional interpreter typically runs much faster.