It's pretty well known that WiFi on the RPI3 wasn't very good. Sure it works but if your RPI3 is in a location with marginal WiFi signal an outboard USB-WiFi dongle/antenna will often help.
I had to use a WiFi dongle/antenna on my RPI3 when I had that setup in garage quite a distance and several walls from my WiFi router. With WiFi (RF) it's not just about distance as everyone's home, walls, and other obstructions and reflections differ. Tons of variables to consider that differ from home to home.
