After installing Uniform Server for the first time and opening up phpmyadmin, I see that there are 3 databases that already exist. I didn't delete them as they must be there for a reason.

• information_schema
• performance_schema
• phpmyadmin

Why  are these databases existing, and what are they for?

Those are the default base databases required for the functioning of MySQL and PhpMyAdmin.

INFORMATION_SCHEMA provides access to database metadata, information about the MySQL server such as the name of a database or table, the data type of a column, or access privileges. Other terms that are sometimes used for this information are data dictionary and system catalog.

From https://dev.mysql.com/doc/refman/8.0/en/information-schema.html

The Performance Schema provides a way to inspect internal execution of the server at runtime. It is implemented using the PERFORMANCE_SCHEMA storage engine and the performance_schema database. The Performance Schema focuses primarily on performance data. This differs from INFORMATION_SCHEMA, which serves for inspection of metadata.

From https://dev.mysql.com/doc/refman/8.0/en/performance-schema.html

PhpMyAdmin is a PHP application that is used to manage the MySQL Database. The phpmyadmin DB is where the data related to that application is stored.