Q:

the meaning of multiple threading on single core cpu

Once I thought the only occasion multiple threads should be used is when IO processing is needed.
But I heard it's also useful without IO processing. Because it helps to occupy more CPU resources.
In my understanding, this would be 
the process with more threads are given more CPU time.

Is this why multiple threads help improve performance even on single core?

A:

One possible reason you can see greater performance from multiple threads on a single CPU is that CPUs tend to be really good at instruction reordering and making use of instruction-level parallelism.  Threads have fewer data and control dependencies with respect to one another than any two sequential instructions within a single thread, and therefore they offer more possibilities for the CPU and OS-level schedulers and re-ordering mechanisms to be very clever.  
Don't forget that things like "reads and writes in memory" are still "I/O" when viewed in a particular way.  These are relatively slow operations, and much of the pipelining in modern CPUs is used to hide memory latency - having multiple threads executing at once can be useful for filling up time that would otherwise have to be filled with delay slots where there are data hazards within a single thread.
That said, threads are often not a good solution to increase performance, and can have precisely the opposite effect.  It can be very easy to saturate all available memory bandwidth using a single thread on some problems.

