This is the first pet tier; it has pets with the lowest multiplier.
When you began the game, you were offered the chance of a cat or dog - this is the tier at which those pets are available.
If you have very little coin or are just starting out, get three of these pets. If you get more, you will be able to select the tier-01 pets that have the highest multiple.
Early in the game, you have a very small back-pack and a single pet - thus, each additional pet past the first will have a massive augmentation effect on the coins you receive when selling the contents of your backpack.