Buy It If You Want Access to the Battlefield 1 CTE
If you want access to the Battlefield 1 Community Test Environment (CTE) you'll need to buy the Premium Pass.
The Battlefield 1 CTE allows you to test future updates and upcoming DLC before they are officially released to the public. DICE put its Winter update through testing in the CTE and They Shall Not Pass went through the CTE on its way to release.
As of right now, the Battlefield 1 CTE is limited to Premium Pass holders and Windows PC users. DICE is planning a release for PS4 and Xbox One and if everything goes according to plan, it should be out for console sometime this month.
If you want to help DICE test for bugs or if you simply want to get hands-on with updates and DLC ahead of time, you'll want to think about buying the Battlefield 1 Premium Pass.