Blender is able to use your computer's central processing unit (CPU) or graphics card (GPU)—or both—for rendering. To eliminate render wait times, you might consider investing in multi-core processors and GPUs with a high CUDA core count.

Does Blender need GPU?

In short, No, you do not need an external GPU. Not to get too technical but except for drawing and viewport stuff amongst some other things, much of Blender's core functionality relies very little on a fully fledged GPU.

Can Blender use both CPU and GPU?

On the performance side, there are also significant improvements, and a notable addition is the new hybrid render mode. This allows rendering using both the CPU(s) and the GPU(s) from the same machine, promising speed improvements in all Cycles renders. The hybrid rendering mode is indeed an interesting feature.