Why there is a partition type in GUID Partition Table?
What are they using for?
Because partitions hold different types of data. Here's a list of some of them.
Operating systems use the type code to decide whether a partition is one that it's interested in. For example, Windows only creates drive letters for partitions with specific types that it's aware of; it'll ignore partitions belonging to other operating systems, since it doesn't expect to understand them. And EFI firmware looks for a partition with a special "system" type code and uses it for booting.