Multiple ground & power connections will lower high frequency noise when the digital signals switch. When a digital pin changes voltage, a high frequency current flows for a brief moment as the stray capacitance of the pin, wires and any other pins or connected parts is changed. Ultimately the current flows through ground wires, which form a loop. The larger the area of the loop, the higher its inductance is. Rapidly changing current through an inductor causes a brief voltage spike. This can be seen as ringing or other artifacts if viewed on a high bandwidth oscilloscope.
By default most pins on Teensy 3.x are configured with slew rate limiting, which greatly helps reduce high frequency noise and lessen the need for low-inductance ground connections. But some faster signals like SPI and I2S clocks are used without slew rate limiting, because of the impact it has on their speed. Closer and redundant ground connections help most in those cases.