Profit to the company is the simple answer, and is not and should not be a dirty word.
A lot of smaller companies cannot afford the larger "single fee" optimizations. Providing a monthly billing allows them to stretch it out over a longer period of time, and ensure that any new content they create is optmized as well.
There is another benefit for the end customer in that (in theory) they will be provided with on-going support. Whereas most providers I know (be it web designers or optimizers) Will unfortunatly be "unavailable" to their previous customers after the changes are "live". Unless there is another charge or fee made.
A month to month system will also ensure an improved relationship, lend itself to increased referrals and better word of mouth.
I think too many people (not the writer persay), but many in this business are still treating it too much like a Hobby, than a real business with real business goals.
