If your employer allows you to pay for development software, they can get into legal issues.

Companies (depending on jurisdiction) are not allowed to make employees pay for items necessary for work.

And allowing employees to pay can easily be misinterpreted into subtly pressuring employees to pay.

This is usually because the software you pay for yourself has licensing terms that don’t allow commercial use or reimbursement. See the JetBrains personal license terms itself. Understandably, the company will find it easier and cheaper to forbid use than hire a lawyer to check each license for each software that each employee wants to use.