It's not just raising prices - it's holding prices steady at some point without the concurrent pressure to sell, for example, or manipulating other markets in order to raise or lower prices in an area, or using other mechanics to manipulate pricing, across the entire market, depending on the intended actions. If they intend to purchase properties, it benefits them to depress pricing in the area, if they intend to rent, they can afford to impose artificial scarcity until they force renters to meet their rates, and so on.
Normal landlords don't have effectively infinite money with no forces bearing prices down, nor do they have the capabilities to influence markets. Even tiny percentage shifts can result in significant fluctuations in the prices consumers see. It's a very nuanced and complex system in which these institutional investors have very outsized influence.