In electrical work, what’s the point of a red wire if it has the same purpose as the black wire?

 In electrical work, what’s the point of a red wire if it has the same purpose as the black wire? Why not just have two black wires?

You can. Nothing wrong with that. But it lets you keep them separate, is you choose to. For wiring an electrical clothes dryer, range, or water heater, it doesn’t matter which is L1 and which is L2 (until you’re troubleshooting a problem).

Black and red are what US electricians expect to see in a 240-volt circuit (although, again, two blacks isn’t wrong) but I’d tape one of them to be red to make it clearer for the next worker.

And for traveler wires between 3-way switches and light fixtures, it’s nice to have different colors (as is found in 14/3 NM-B). Likewise for smoke alarms that are wired together - you need an additional conductor beyond hot, neutral and ground and 14/3 conveniently gives you a third insulated conductor in a another color.