There is a difference between praising effort when effort is needed, and praising pointless effort that is just self-punishment.
Later in life children will be put in situations were being unable to work effectively and put in effort will have really bad impacts on their lives. Preparing them to be able to cope is for their own best.
That does not equate to teaching them to be good little worker drones regardless of their own happiness.
> kids already have great deal of intrinsic motivation why do we need to praise him.
Because not receiving praise and positive attention is soul destroying and leads people to seek other ways of getting attention. In the absence of positive attention, any attention - including punishment will do.
Got a kid? Try ignoring them, and see attention-grabbing behaviour escalate dramatically in an amazingly short time span. From my son was 2 and to this day (he's 7), withdrawing attention has been our primary means of discipline, and it is scarily effective. When he was 2-3 we'd limit it to ignore him for 20-30 seconds at a time.
(In fact, it works with adults too - just try shifting your body away from someone to punish behaviour you find rude or offensive. It's crazily effective with most people)
So given that you almost certainly will praise your children if you want them to thrive, the question is what to praise to help them do as well as possible.
Praising them for effort does not mean praising them only for that, or praising them for working too much. It means praising them for effort in preference to praising them for results. But also praise them for other things, including ensuring they get enough rest and take time to play.