To enhance their looks
Many filters are designed to enhance people’s looks and make them seem more attractive. People use such filters because they allow them to post better looking photos.
To enhance their self-esteem
Many people feel better about themselves when they post attractive pictures. Many filters give people a self-esteem boost because they make their pictures seem more attractive.
To change skin color
Many people use filters to either lighten or darken their skin color. Because filters change the lighting settings, they allow people to make their skin color look different.
To get better engagements
According to a study, filters can result in better engagements, more views and comments. Many people might be motivated to use filters to get higher engagements.
To hide imperfections
Many people use filters to hide their imperfections and make photos look more flawless. Many filters are designed to hide common imperfections.
To make the photos seem professional
Many people use filters in order to make their photos seem like the ones taken by professionals. Prisma app, for example, makes ordinary pictures look more artistic. (See What makes a person attractive?)
To adjust lighting settings
Filters allow people to adjust lighting settings without having to take any additional steps. This makes the process of producing a better picture much easier.
Giving people more options
Filters give people many options to choose from, and as a result, they can help them come up with the best picture possible.
Many popular apps promote them
Many popular apps such as Instagram and Snapchat ask people whether they want to use filters or not. Many people feel like trying those filters, and as a result, end up getting used to them. (See Why is Instagram so popular?)