I am a fresher learning essbase and Hyperion, in one of project there was a outline in essbase where level 0 members were tagged as never share, I just don't understand why they are tagged as never share when they don't have children under them which will make implied sharing happen??
You're on the right track regarding implied sharing. I don't think the Never Share option was used a lot before the advent of Hyperion Planning, but when Planning came to be, they started marking members as Never Share by default so that people wouldn't accidentally stumble in to issues where implied sharing was happening on parents with just one child and data was being entered into a form. Then, if you eventually go add children to the member so that it becomes a parent, you won't have implied sharing issues.
There isn't much harm in having Never Share set across the board and treating it as the default member property. Also, given advancements in cube technology and hardware, there is precious little to be gained by saving a few bytes with an implied sharing relationship anymore, especially if it prevents some difficult and time-consuming issue to troubleshoot with your data that happens as a result of an implied sharing relationship.