Here's the deal. Look at you heat sink. Looks pretty flat doesn't it. Well it's not really all that flat if you look at it microscopically. And the sink on the chip is not all that flat either. There are little gullies and hills where the metal is going to be separated. Where the metal touches metal the heat transfer is great. Where the metal just almost touches the heat transfer is less than perfect. By putting on the thermal grease you fill the gaps and "bridge" the heat transfer. It's still not perfect but much better than before. Where the pieces fit metal to metal, the grease is squeezed out.
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Yes, but as Framistan says, and I emphasise, a VERY thin (translucent) film. You WANT to get pure metal to metal contact, and only a few points where the paste actually does anything.
on components that get hot, yes. You dont need a lot of it. Just a thin layer. If you apply too much, it will just squeeze out when you tighten the bolt holding the component to the heatsink.
If the component mounting calls for it, yes. There are exceptions, but without knowing what you're using and what you're using it for, I couldn't say much more than yes.
No, but your CPU or whatever device you're trying to keep cool will most likely run substantially warmer than it would with the thermal paste. Possibly, to the point of overheating.