When the natural behavior of a plant or animal results in a benefit to people, that’s an ecosystem service. One example is that plants clean the air as a side effect of their respiration.
Another example is water absorption and filtration. A large tree will absorb hundreds of gallons of water during a rainstorm, preventing that water from ending up in storm drains or basements. Like with air, plants filter pollutants from water as they absorb it. This lessens the burden on municipal water filtration, saving a city money and improving local water quality.
A third important ecosystem service is pollination. Bees and butterflies pollinate crop plants as they visit them to gather food for themselves. Without such visits, virtually all crops aside from grains would fail to produce anything. The plants would need to be pollinated artificially, which would make food much more expensive.
Technological replacement of ecosystem services is expensive in general. If we had to provide these services ourselves, the annual price tag would be equal to twice the total wealth of the entire world. Since we can’t pay for these services, and since civilization as we know it wouldn’t function without them, it’s critical that we continue getting them for free from healthy ecosystems.