OKRs, on a team, and company level make up a system that shows how everything one person does connects to the work of others.
If an employee knows that not meeting his goals makes achievements harder for people in other departments, they will want to try harder. When everyone knows how their work matters, it increases overall engagement, motivation, and determination. It’s a psychological effect: no one wants to be the weakest link, so they’ll try harder. This is a very powerful tool.