Each zodiacal Sign represents one element.
To find out why Fire, Earth, Air, Water, see notes on Zodiac. The elements sequence repeats 3 times. The lion is fire and the pisces fishes is water. This is how the animals have been selected by Indian sages thousands of years ago.
For predictions the elements indicate properties. If Leo is my house of occupation [10th], then my job involves fast changing fire like items - computer software.