Stages are used to group parts of the process into logical steps as well as reporting. For example, a travel request process could have big picture steps like "Manager Approval", "Travel Coordinator Approval", etc. As part of one of those, there could be multiple activities, say, in the "Manager Approval" stage, like sending the form to the manager, sending it back to the submitter for revisions and so on. For a process standpoint, the user would not necessarily be interested which activity is currently running, but rather that it's still going through manager approval. This also allows you to do high level performance metrics and see how many tasks are waiting in each stage, whether the approver  at a given stage is overloaded or constantly late in completing the task and so on.

Thanks for the prompt reply, sorry for being so ignorant, now i do understand what actually stages are for but now next i need to know how do i connect these different stages together to get these processes stay connected.

Reference to the last reply by Miruna, if i divide my process into two different stages first Manager Approval Stage second Travel Coordinator stage how do i get them connected on the design prospective on the process modeller, this is what i fail to understand as i haven't used it ever but it does seems to be interesting to me and its very beneficial for the users to monitor the performance of all their process individually.