Using a limited palette really focuses one's mind, and skills, and I tend to use only 3 base colours with a few other colours for accents or contrasts.
It helps to keep unity in a painting, and that amongst a few other devices is easily achieved with a limited palette.
I use primarily Cobalt Blue, Permanent Rose, and Cadmium Yellow in all of my work and augment that with about 4 other colours in small varying amount.
This painting was done using only Cobalt Blue, Permanant Rose, and Cadmium Yellow, and I feel that helps it hang together cohesively.
It's a fun way to learn to mix colours and learn about their individual properties, plus you can then juxtapose your limited colours against each other to maximum effect.