Jumping from lab scale straight to full production can result in problematic mixing, product output and slow or hard-to-control reactions. Chemical processes do not scale linearly, and this can make it difficult to predict how a full-scale commercial process will actually behave.
While some issues can be addressed in simulations, the physical version that runs in the real world will often still behave differently than the simulations predict. Modeling every factor takes time and may be too complex.
The realities of physical system layout and equipment constraints also play a large role in pilot plant design. Non-uniform concentration gradients can cause less than ideal behavior and invalidate some assumptions.
A pilot plant allows you to collect real data that can help to ensures your full-scale production plant runs properly. It allows you to experiment with inputs, outputs, processing time, etc. to streamline your process. Pilot plants can be used to: produce usable product, start building a market, and convince investors of long-term viability.
In some cases, a pilot plant may be the optimal size to produce specialty products in low quantities. You can stop at the pilot plant scale, produce plenty of product, and forgo a larger financial undertaking until the market demand increases.