A padlock is useful for when you have valuables that you just don't want to see disappear. It could for example be your bicycle, your things in the gym locker and so on. There are multiple options on the market and you buy one based on what you need it for. If there are multiple people that have access to the padlock then it might be best if it's one with a combination code. That way it's you that decides who can open it, and if you have small children there's no risk of them losing or throwing the key away.
If you are looking to buy a padlock you should have in mind what you're going to use it for - some are more suitable for indoor use, while others can withstand outdoor weather conditions.