What is the benefit of using a SEO service provider?

This question can be briefly answered by adding 4 fronts:

1. Management
2. Workflow
3. Economics
4. Fiscality

1 - MANAGEMENT

Using a SEO service provider allows your company to focus on what’s really the core activity -the kind of things you do well and really know about- and use your work time on the most added-value for your profit or goals. One of the things that make a good manager is knowing when to delegate.

2 - WORKFLOW

Using a (good) SEO service provider improves timings and impact.

This provider will know way better than your own resources what to do, how and when to do it and getting the whole thing seamlessly integrated into your daily processes.

This processes will loop sooner or later, then management can measure and dictate if the provider is an investment or an expense.
Investment equals longer term collab, expense means "next, please!".

3 - ECONOMICS

Using a SEO service provider makes complete sense from a macro-economy perspective.

I am talking about the basis of capitalist economy: specialization.

Simplifying things: if you can make a product, say t-shirts, and you are at the same time bulking resources on self-SEO, then a pure economist may say you are not as good as you could really be at your business.

This is highly argumentable, since big brands can build in-house teams which will revert into a highly skilled SEO team plus deep knowledge of the niche and full dedication.

My experience says they always do once they cross Rubicon.

From experience I can also tell that companies with internal SEO service will call for help again for very specific missions.
Regarding these missions, in my professional case I am usually being hired for specific Structured Data (Schema) missions even from companies I would not bet were in need for SEO help.

4 - FISCALITY

Using a service provider of any kind is generally advisable for fiscal policies, as inbound invoicing helps reducing taxes (upon each country fiscal law).

Doing things on your own may mean creating workplaces, which is obviously good, but may create a whole new structure cost.

An active SME is probably preferable to a large and dead corporation...