Peter Mead has an insightful blog post on the advantages of series preaching. In sum, Mead notes five advantages.
A series of sermons has greater leverage than a solo sermon.
A series of sermons can create momentum beyond the moment.
A series of sermons allows messages to balance each other.
A series of sermons allows for longer lead time in preparation.A series of sermons allows for overlapped or deeper exegetical work.
Make sure to read the entire post by clicking the link above.