How smartsheet developer tool account is different from paid account. Also how developer account can be use for API call as pre-production environment.
Developer Tools can be enabled on any Smartsheet account. Having the Developer Tools enabled allows you to create an app for implementing the OAuth flow.
The purpose behind having a separate Developer account is to ensure you can experiment with the Smartsheet API without worry of making changes to critical items needed for every day work.
Since the Developer account is signed up with a different email address than the one you use for your main Smartsheet account it is a completely separate environment. But, it still acts like any other Smartsheet account. Without sharing the account to items in Smartsheet it won't be able to access them. This way you can create Sheets, Reports, and Dashboards to simulate the data you will be working with. Then as you make API calls to them you know you aren't causing issues for the real items people at your company are working on in Smartsheet.
One approach would be to share the Developer account to a collection of things you will eventually be running your requests against in production from your main Smartsheet account. Then in the Developer account make copies of those items. Once this is done you can un-share the Developer account so that it no longer has access to the real items in Smartsheet. Then you can make requests to the API to test your code and see the results as it would really happen. Once you've proven your code works you can adjust the access tokens and id numbers of Smartsheet items to work for your production stuff.