The first term of every presidency is largely spent running for the second one. Fundraising starts immediately and policy gets filtered through re-election math. Decisions that might cost votes get deferred until after the next election, which means they often get deferred indefinitely.
A single six-year term removes that distortion entirely. The President governs for six years, then goes home. There's no second campaign to run, no donor base to maintain, no calculation about what plays in Iowa.
Presidential elections become less frequent, less expensive, and less all-consuming. It's a structural fix to a structural problem.