What are the benefits nowadays to use a Router instead of a L3 Switch ?
It usually comes down to options and resources, where routers have more of each. Routers often have the ability to have different types of interfaces that are not available on switches. They also typically have more resources (RAM, processor, etc.). Routers also usually have a hardware assist for NAT, which is actually resource intensive, and most switches do not NAT. Also, routers usually have an option to run a firewall that many switches do not because firewalls are resource intensive.
In general, you use layer-3 switches as LAN routers, and use a router for WAN connections.