As stated in the docs, the starter one is a convenient inliner for all required dependencies for this particular library, i.e. includes other dependencies in itself, instead of you writing those manually.
Look into the spring-boot-starter-data-jpa pom.xml, you will see there it includes spring-data-jpa as a dependency among many others.
Spring Boot Starters are a set of convenient dependency descriptors
that you can include in your application. You get a one-stop-shop for
all the Spring and related technology that you need without having to
hunt through sample code and copy paste loads of dependency
descriptors. For example, if you want to get started using Spring and
JPA for database access just include the spring-boot-starter-data-jpa
dependency in your project, and you are good to go.