Free tiers are great for trying things out and casual use. Paid tiers — typically around $20/month — get you the latest and most capable models, longer context windows (more memory per conversation), file uploads, and advanced features like agentic browsing and code execution.
If you use these tools more than a few times a week for real work, paying is almost always worth it. If you're using one occasionally to draft a birthday toast, free is fine.