What's the purpose of the ground plane?
A ground plane:

provides a low-impedance ground connection,
This is important since &quot;ground&quot; is how different components relate to each other. When ground lifts because of impedance noise margin is reduced and errors can result.

acts as a EMI shield,
If your product needs to undergo any sort of official certification then you can know that half of it is already shielded and you only need worry about the other half.

and can act as a heat sink.
This is not that important for small-signal components, but any sort of power electronics, or even just a voltage regulator for the rest of the board, can benefit from this.