With their ability to fly, birds have many advantages over earthbound animals. But flying takes a lot of energy, or fuel. And the bird has to get this fuel from his food sources. Food is fuel to a bird, much like gasoline is fuel to a jet airplane.
But, unlike earthbound creatures, birds that fly have to be finicky about the foods that they eat. The food can not be too bulky else it would add unwanted weight to their bodies and thus hinder their ability to fly. The types of foods that are best for birds are foods that are small but yet dense in nutrients. A food like seeds.
Among the most nutritious and nutritionally dense foods available to birds are seeds. As a matter of fact, most seeds are so filled with nutrients, that, as long as there are seeds around, a lot of birds will eat nothing else. On the other hand, if seeds are hard to find, then many birds might very well switch to secondary sources of foods such as insects, fruits, and so on. Most of these substitutes don't pack the condensed energy value of seeds, even though they are valuable food sources.
During the millions of years that birds and plants have coexisted on the planet, many plants have seen their seeds consumed by billions of birds for their nutrition needs. And they have continually developed defenses to thwart them. One type of defense has been to develop a hard shell around the seed which cannot be breached without tremendous efforts. In response, some of the birds that feed on these seeds have developed extraordinary strong beaks with the capacity to exert hundreds of pounds onto a shell that easily crack it.
Other plants have taken an alternative approach. They have evolved with spiny needles around their shells which prick anyone who attempts to extract the seed from within. In response to this, many birds developed special slender beaks that enable it to avoid the spines and reach the seed inside.
Other plants have developed seeds with extremely unpleasant tastes to dissuade birds from wanting to eat them. Unfortunately, most birds have a very limited number of taste buds in their tongue, so the taste doesn't really bother them that much.
And, still other plants have developed poisonous seeds. In response, certain birds have learned to ingest foods which neutralize the poisons along with the poisonous seeds. This way they remain unaffected.
Over the centuries this has resulted in a near constant state of warfare as plants evolve with defenses to protect their seeds from birds and birds evolve to counteract the most recent plant defense. This, in large part, accounts for the huge number of beak styles that you will see within various bird species.
Many veteran bird watchers are often able to tell a lot from a bird, even if they are unfamiliar with it, by simply looking carefully at the shape and structure of its beak.