Eli5: Some ice cream recipes put ice + salt outside the recipient to make it cool faster. But in the winter, salt is put on snow on the street to melt faster. Why one make cool and other melt?

Question

Edit: thank you all for the explanations, I now have understood much more!

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Answer 1

It’s been described accurately here, but maybe a way to visualize it could help too.
As people have stated, salted ice melts faster than non-salted ice because the freezing point is lowered. Why is quickly melting ice good for making ice cream? Well, in order for ice to melt (salted or non-salted) a lot of heat needs to enter that ice to break the crystal water formations. Like, a LOT of heat.
That energy has to come from somewhere, and unlike outside (where the sun can help), your ice cream maker ice has only one option – steal it!
So the energy hungry salted ice gobbles up all the heat from every nearby object – and freezes your ice cream solid.