Heat Sinks become available right after Heat Exchangers. The primary purpose of this component is to contain wast amounts of heat. It is a good safety vent, as sink dissipates five percent of it`s current heat each tic, so as long as heat surplus does not exceed the five percent of sink`s max capacity (meaning 200000 heat initially), sink will keep the network from going boom.
However, use of heat sink is inefficient, as the dissipated heat is not monetized in any way. This may be partially offset by attaching Boiler Houses to the heat sink to sell off the excess heat.
As a rule of thumb, heat sink is not necessary on the well-balanced field, but can serve as a convenient safety valve either by being rapidly placed into overheating grid, or by being placed into the grid at the end of generator chain to suck up all the heat leftovers.
IMPORTANT NOTE - only boiler houses can pull heat out of heatsink. Placing generators next to one is, at best, inefficient.