Modern high-speed internet uses light to quickly and reliably transmit large amounts of data through fiber-optic cables, but currently, light signals hit a bottleneck when data processing is necessary. For that, they must convert into electrical signals for processing before further transmission.
There are such things as L1 switches, they are not really switches in the Ethernet sense but rather more like crossbar switches. They can selectively connect all traffic from two arbitrary ports together, and then change that on the fly, without a wiring change. Applications that are obsessed with getting the lowest possible latency might opt for those.
Ah, yes, high-speed stock trading. One of the big drivers of network innovation.