Some, like humans, bears, etc., are omnivorous, and it isn't clear where they fit into such a model. Still, it is a useful simplification, and gives us a general idea of what happens. How some toxic compounds that don't break down easily get accumulated at higher concentrations as one proceeds up the food chain. To understand questions such as why don't ecosystems run out of the materials they need, we need to know how much is in each component, what the rates of transfer are from component to component, and we need to understand the factors influencing those rates. The global water cylcle provides a simple example.
Evaporation moves water from land, and from lakes, rivers, oceans, into the atmosphere.
Precipitation moves water the other direction. Runoff moves water from terrestrial systems into the oceans. So evaporation is a way of moving energy from the surface into the atmosphere. Some water is stored as ice in the polar ice caps and alpine glaciers. This amount depends on climate.