From my understanding there are two main beneftis:
- Capacity
- Safety
Roundabouts work great, until the amount of traffic becomes to big. Then it actually starts causing problems.
At that point you can put in a regular intersection with traffic lights, which actually works better than a roundabout does in high traffic environments. But you do lose out on the traffic safety benefits, with head-on collisions becoming possible again.
A roundabout with traffic lights increases the capacity of the intersection while still reducing the risk of deadly accidents.
It's also a lot cheaper than upgrading to the next step, which is building an interchange.
Signalised roundabouts are also quite prevalent in the Netherlands, and I can speak from experience that they generally work quite well if the lights are adjusted properly.
Note: I'm just some random guy, I'm by no means an expert on the matter.
This is just my understanding of the benefits of lighted roundabouts.
I suspect it's one of those topics he likes to drum up if he needs to distract from something else.
(My suspicion is that this other topic is the walking back on campaign promises on immigration with the H1B visa thing)
That doesn't mean it's not still harmful. This stuff does harm relations with America's European allies, but I suspect that he is not actually planning on doing anything.
An alternative explanation could be that he simply wants to get his hands on a very resource rich area such as Greenland.