SEO consultants are worth it if you have a good existing knowledge of SEO and have the time to work on tasks yourself (or within your team), but just require a bit of strategy, guidance and support.
They will drive your campaign in the right direction without spending time executing work. This is particularly valuable if you know that you are capable of doing the work, but don’t really know what that work should be or in which order to do it for the most impact.
