The purpose and implementation of cond_resched()

I have a question about cond_resched().

What is the condition under which I should invoke cond_resched() irreplaceably?

For example, I see the following code in ksoftirqd(),

preempt_enable_no_resched();
cond_resched();
preempt_disable();

But I do not understand why I should not write the following code,

preempt_enable();
preempt_disable();

Are the above two pieces of code equal in functionality?

On the other hand, I see cond_resched() check and set PREEMPT_ACTIVE.
I currently do not understand why it should do this, since I think
PREEMPT_ACTIVE is only used to be set in the return-from-interrupt
code in order to prevent schedule() from removing task from run queue
unpredictably. But for cond_resched(), which is a planned voluntary
switch, why does it also deal with this flag?
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