The advantage of succession planning for all levels of an organization is that such planning ensures continuity, the presence of qualified replacements, and smooth succession (or at least makes it more likely) at all levels, not just at the top.
There are at least two main reasons to engage in succession planning. First, it is important to be able to replace people as quickly as possible when positions come open. It is important not to leave a job vacant for a long time. Second, it is important to make sure that there are qualified people ready to step into any openings. If this is important for the highest positions in a firm, it is also important for positions at the lower levels.
Much of the work of a firm goes on at the lower levels. While these employees do not add as much value as the top management does, it is still not possible for the firm to do without them. Given that they are important to the firm, the firm should develop succession plans to replace them when they leave the firm or move up the ladder.
The advantage of succession planning at all levels, then, is that all levels will experience smooth succession processes, thus allowing the firm to continue its work as efficiently as possible.