On an Arduino Uno, pins 9, 10, and 11 are part of a specific group of pins with dual-purpose functionality:
Using certain libraries (like the Servo library) can disable PWM functionality on pins 9 and 10 because the library "takes over" the internal Timer 1. Pin No 9 10 11
These three pins (9, 10, and 11) are frequently used as the address lines for 4051/4052 analog multiplexers to expand the number of sensors you can read. On an Arduino Uno, pins 9, 10, and