By soldering capacitors across the motor terminals, you help suppress the noise by smoothing out the voltage spikes. The motors that lack capacitors might either not require them due to their design or might simply have omitted them, but adding capacitors can improve performance in noise-sensitive projects.

1) When you spin a DC motor, it actually tries to fight back a little bit by generating its own voltage, called back EMF. This can cause some voltage spikes when the motor starts or stops. The capacitor helps absorb those spikes and keep the power supply happy.

2) Brushed DC motors, can generate sparks as the brushes switch between contacts. This sparking creates high-frequency electrical noise that can mess with nearby electronics and even cause radio interference (EMI). The capacitor acts like a tiny little battery, absorbing those quick bursts of current and smoothing things out.

Now coming to the question, why don't all DC motors have them?

Not all manufacturers include capacitors in their motors, especially in low-cost or basic motors where cost and simplicity are prioritized.

In some cases, you are expected to add your own capacitors based on the specific application needs.