As it currently stands, NATO is dominated by the US, both in terms of equipment and procedural standards and in the control structure and strategic outset.
The US has become an unstable wild card since Trump, if not already being one latest since the illegal and failed Iraq invasion and the two decades of failed Afghanistan "intervention". Imagine Trump gets into power slashes all support to Ukraine and blocks all weapons exports to the EU, if they could be used to defend Ukraine...
The EU needs strategic autonomy from the US. In particular looking at the refugee crisis that has been strongly exacerbated by the US work to continuously destabilize West-Asia (Middle East), it shows that US interests often are directly opposed to EU interests.
At the same time, creating a military union in the EU requires its own command structure. Building that up will take some of these structures out of NATO, but if NATO is truly an alliance of equals, it can handle these changes. If the EU wants to persist in a multi-polar world, it needs to achieve military autonomy too.
I hate Macron, and find practically all of his policies terrible. This is the only topic where i think he has some strategic foresight and i can agree on with him.