1. Why it is paid (when web interface is free) ?
2. What are there benefits of buying iPhone App ?
3. Is toodledo iphone app is web interface replacement ?
4. Do i need to purchase subscriptions plans or it will support free version ?
1. It is paid because TD is a for-profit company, and like many for-profit software companies, they use the free online version to lure customers in, hoping that enough will upgrade to a paid version so they can stay in business and make a living at it. Otherwise, it will just go away.
2. You'll have TD on your phone, and no longer have to be at your computer to use it.
3. The iOS app is not a full replacement. You can add tasks and see your list, and many other things, but you won't have lists or outlines and many of the more advanced features aren't available as yet.
4. It will work with any version.
You will always get an answer here, if not from one of the members, who are very helpful, then from Jake, the owner developer, who answers virtually information requests such as yours, in addition to fixing the many mistakes some of us make in our replies. :)