Because somewhere, somebody must start to resist the global power grab by the leftist elite.
That is exactly what these Yellow Vest protests are: resistance to Globalism. That is my opinion. It is having an effect around the world. However, I think Trump started it. Brexit fits in there somewhere.
Need to encourage/support them.
France had a Socialist government for five years before Macron, so I don't think the yellow vests particularly mind leftist governance.
Polls also show that support for Marine Le Pen's new party, the Rassemblement National, topped out in December at 24% and the average Frenchman is tiring of ongoing civil unrest.
This means there is great potential for the yellow vest movement to back fire politically on the French right...which I'm sure is what Macron is counting on.