My N=1. You can create a decent tri/TT position on a road bike, but doing so does involve shifting things around. Typically moving seat forward and raising it, and dropping the bars, to get a steeper and lower position which is more aerodynamic without compromising your hip angles and power output. This is fine if you just want to do the occasional event or give it a go to see if you like it.
If you find you want to compete regularly in non-drafting triathlon or TTs, then regularly switching your bike between road and TT setup becomes a PITA, and it’s worth investing in a tri/TT bike. That way you can head out the door (or onto the trainer) for a road or TT session whenever you want without having to get your torque wrench out, plus you can get the incremental gains of having a bike that is designed around TTing (e.g. a more aero frame, deeper wheels, cleaner cockpit, TT shifters, steeper geometry, etc). And 2 bikes are always better than 1!