Aside from letting you cram more circuitry onto the same size chip, smaller transistors means you can get better power efficiency and reduce heat output.

Basically, even if you just take an existing design and use it to make chips at a smaller node size, you get chips which run cooler and with less power. Those chips can then get you the same performance with better efficiency (e.g. same speed but better battery life), or you can crank up the speed so that you get more speed for the same amount of power as the original.

And as mentioned above, because the transistors are smaller, you can fit more stuff onto the chip. So you can make even more complex chips which also still run more efficiently than their predecessors (both because of the direct power savings from using smaller transistors, and because designs become more efficient).