It is not wasting space but making design better.
At signal lines you have pulses. Pulses contains lot of higher frequency harmonics. To minimize emitted by PCB disturbances and to minimize sensitivity to interference (from external sources and also from neighbouring lines) it is crucial for each signal line to have as close to it GND connection to allow that line current to get back to the source the lowest possible impedance way.
For example when I place at PCB the microcontroller that has many VCC+GND pin pairs I give each pair a capacitor and connect each GND pin to GND and all VCC pins together.
At the center IC on the following picture you can see 6 such constructions (2 at top, 2 at left, 1 at bottom and 1 at right):