Why does Java have an IINC bytecode instruction?
There is already an IADD bytecode instruction that can be used to accomplish the same.
So why does IINC exist?
Only the original designers of Java can answer why they made particular design decisions. However, we can speculate:
IINC does not let you do anything that can't already be accomplished by a ILOAD/SIPUSH/IADD/ISTORE combo. The difference is that IINC is a single instruction, which only takes 3 or 6 bytes, while the 4 instruction sequence is obviously longer. So IINC slightly reduces the size of bytecode that uses it.
Apart from that, early versions of Java used an interpreter, where every instruction has overhead during execution. In this case, using a single IINC instruction could be faster than the equivalent alternative bytecode sequence. Note that JITting has made this largely irrelevant, but IINC dates back to the original version of Java.