Why it is bad to use GUID on primary key?

The main purpose of a primary key is to uniquely identify each row in a table. Using a GUID as the primary key achieves this effectively. However, it's important to know that creating a primary key also creates a unique clustered index by default.

A clustered index determines the physical order of the table's rows. Since GUIDs are generated randomly, creating a clustered index on the GUID column can lead to page splits in the data structure. These splits happen when new data is inserted into the middle of a page based on the value of the uniqueidentifier. This can cause problems with INSERTs, UPDATEs, and SELECT statements due to increased fragmentation.