When bacon grease or cooking oil cools in your pipes, it solidifies into a waxy sludge. Boiling water can re-liquefy these fats, allowing them to flush through the system.
If your sink is completely filled with standing water, pouring boiling water on top won't help. The boiling water will cool down significantly as it hits the standing water before it ever reaches the clog. You’ll just end up with a sink full of scalding hot, dirty water. How to Properly Unclog a Drain with Boiling Water can boiling water unclog a drain