There are cats at home all the time, which means it is probably a domestic cat, or there are wild cats nearby who often visit here. There may be many reasons why the cat is waiting at the door. Here are some possible explanations:

1. Wait for the owner to open the door: If this cat is a domestic cat, it may wait for the owner to come home and open the door. This is because they want to go home, enjoy a comfortable life and get food.

2. Waiting to make friends: Cats are gregarious animals and like to make friends with other cats. If you see a cat you don't know in front of your house, it may be waiting for another cat or a group of cats to come here so that they can establish a friendly relationship.

3. Patrol monitoring: The cat is curious and suspicious. They may be waiting at the door, monitoring the surrounding environment and people. If the cat is a wild cat nearby, it is likely to be monitoring potential dangers or looking for food.

In short, there are many reasons why cats wait at home, which may vary according to the situation. If you find a strange cat waiting, you should be vigilant to ensure that there are no doors and windows open or other behaviors that attract cats. If you can't explain the cat's behavior at the door, you'd better contact the local animal shelter or rescue organization immediately to find out the cat's owner or find its new home.