It will be unfair to say that secondary consumers are the most important part of a food chain ... unfair to animals at other trophic levels, as they too play an equally important role in keeping things in order, in both, the food chain and food web. However, animals featuring at this trophic level do have some unique features that make them stand out. In the food chain, a secondary consumer is an animal that feeds on primary consumers. An accurate definition, however, will be an animal that is found at the second trophic level in a given food chain. While primary consumers exclusively feed on plants (producers), tertiary consumers feed on animals (primary and secondary consumers). In contrast, most secondary consumers prefer animals (primary consumers), but some of them alternately feed on plant products (fruits, berries, etc.) as well. So, this group of animals comprises both, carnivores and omnivores types. A typical food chain comprises producers, primary consumers, secondary consumers, tertiary consumers, and even quaternary consumers, at times. It begins with plants resorting to the process of photosynthesis to harness the Sun's energy and convert it into food. Only plants (autotrophs) have the ability to pull off this feat―other than certain species of bacteria―and thus, consumers (hetrotrophs) are left with no option but to rely on them for food. That includes secondary consumers as well. The food chain is important from an ecological point of view, as it facilitates the transfer of energy from one trophic level to another. Secondary consumers derive energy from primary consumers on whom they feed. The specific prey of a secondary consumer will differ depending on the animal in question, place, food availability, etc. Herbivores and small carnivores form a major chunk of their diet. Eventually, when these predators become prey, the energy stored in their body is transferred to the tertiary consumers. Secondary consumers fall prey to larger carnivores, like lions, tigers, leopards, wolves, etc. And if they survive these predators and eventually die a natural death, then detritivores or decomposers work on their remains, and the food chain completes a full circle. If you go by the definition of a secondary consumer, which states that it's an animal that feeds on primary consumers, you will realize that there is no dearth of examples out there. These include lions from the African Savannah, which feed on zebras and wildebeests; wolves, which hunt caribou in the Arctic tundra; frogs of the tropical rainforests, which feed on a variety of insects; various species of fish, which feed on krill in the marine biome; and so on. It's worth noting that 90% energy is lost at every trophic level during the transfer of energy from one trophic level to another. The remaining 10% is stored in the form of flesh, and passed on to an animal at the subsequent trophic level. So, the amount of energy that reaches higher-level consumers is very low, which is why food chains seldom go beyond 5 or 6 trophic levels. This loss of energy also means that the consumers at a higher level have to eat more food, and precisely for this reason, there are more herbivores and few top-level predators in a given food web.