Do I need a Primary Key if it's already an Index
Table usage patterns are the main driver of if you need them or not. In OLTP workloads, you usually do want them. If you're just dumping data into these tables and reporting off them in big ol' scans, they're likely less valuable.

And while you may not need a Primary Key at all, adding a clustered index can help you avoid problems that arise when tables are Heaps (tables without clustered indexes). 

Again, usage patterns will matter most: if you never update or delete data stored in heaps, you won't hit these specific problems:

Forwarded Fetches


How To Fix Forwarded Records
Identify and Correct SQL Server Forwarded Records
How Forwarded Records are Read and Processed in a SQL Server Heap
Forwarding and forwarded records, and the back-pointer size


Captive Pages


DELETE on a heap table
Heaps, Deletes, and Optimistic Isolation Levels
SQL Server: Inside a DELETE operation in Heaps
SQLskills SQL101: Why does my heap have a bunch of empty pages?