Instead of expecting to ?poach? employees from other companies, consider that your own employees also (want to) undergo development. People often take it for granted that high performers do their work day after day without talking to them about prospects. Identifying high-potentials and high-performers and a culture of appreciation are basic measurements to improve. If you promote employees via internal company advertisements, you show colleagues that it is possible to develop further in your company. This contributes to motivation and to ongoing performance.
By filling positions internally, you can put employees in the vacant position in a shorter period of time. You also benefit from the fact that these employees already know the company and can familiarize themselves with their new function more quickly than external candidates. Finally, you save the costs for external recruitment.