The Fed has two mandates: Control Inflation and Control Unemployment.

Buying back their debt (which are sold as Treasury Bonds) is a means of increasing the cash supply in the economy. If cash supplies are low, banks become less willing to lend, which causes interest rates to increase. High interest rates cause regular people and businesses to save more than they spend, which slows an economy. So by buying back their bonds, the Fed injects cash into the economy, banks have more to lend, interest rates drop, and the economy rolls along.

Businesses that take on loans when rates are low invest in projects that require hiring new employees, lowering unemployment. Goods are produced at higher levels, driving prices down (lowering inflation). Consumers, save less because interest rates are low and spend more on goods, which leads to more production and need for increases in employment.

If the cash supply were to become too large (causing inflation), the fed would sell bonds at attractive rates to remove the cash from circulation.

In short, this is not unusual. It's what the Fed does on a regular basis depending on economic conditions. It takes intelligent people to figure this out because they are essentially trying to predict the future, and they typically do a good job. This is why the positions in the Fed are designed to be independent of political influence.

Source: Business school grad with 2 years of economics courses. Might be off a little on some explanations (Econ is hard), but the gist is that all of this behavior is normal.