Some people create one profile as their private profile and another one for work purposes. They do not want to mix their professional and personal lives, hence create two different profiles.
They are fake accounts
People create fake accounts on Facebook. It means people create a Facebook account in the name of the other person without their consent. (See Why do some people create fake social network profiles)
They do not want certain people to find themselves
Some people do not want other people to find their profiles on Facebook as they want to hide their life and want to prevent others from knowing about it.
They do not want their family to find themselves
Certain people want to hide their accounts from their family as they do not want them to interfere or get to know about their personal lives. They create more than 1 account and add their family members to the other profile.
They are famous
Usually, renowned personalities create one account as their public account and the other as a personal account. (See, How does it feel to be famous?)
To manage pages
Some people who have multiple pages to run their businesses often create a separate Facebook account to manage their pages.
To start from scratch
Some people have so many unknown friends and followers on Facebook that they do not control their profile. So, instead of deleting their friends from the existing profile, they create a new one to start from scratch to know who they are adding to their friend’s list.
They forgot about them
Some people remain inactive on Facebook accounts. These types of people could even forget that they had created a Facebook account in the past. They could create a new Facebook account, forgetting to deactivate the old one.