freezing helps solidify and congeal the grease that degrades and becomes stickier over time and causes the drive mechanisms to stop spinning or blocks the seeking heads. As the drive heats up again, that degraded grease becomes a sticky goop again and gums up the drive.

Heating devices to fix them is a form of "reflowing" where you're getting the solder on the board close to its melting point, helping the solder 'reflow' into the cracks that have formed between its component contacts, via thermal or mechanical stress over time.

But why would grease work better when it gets more solid?

Usually the lighter fractions evaporate away, leaving solid grease that has more resistance.

If you get grease into your garbage disposal grinder, I’ve heard you should use cold water to get it to congeal so the grinder can break it up and it will continue flowing once it’s in the drain. If you use warm water, it might flow, but only long enough to then solidify on your drainpipes.

Maybe something similar is happening on the hard drive: sticky is too viscous to flow at full speed, but solid just breaks apart and dances around on the platters while the heads can make contact with the magnetic media.