If you know what keyword research is, then you will realise the importance of having access to a number of SEO tools that would allow you to achieve your goals. There are several tools related to SEO that are available in the market however they all have different features and different functions as they are designed for different purposes altogether.
There would be SEO tools that would be solely focused on building links as well as those that would be entirely about identifying profitable keywords for you to build content around.
You will also be able to find a good number of tools available in the market that would be all about monitoring your backlinks as well as tools for monitoring your SERP positions for your various keywords that you have been working on over a period of time.
If you choose to invest in all of these tools individually then things could get a lot costlier compared to going for a single service that has multiple tools that you could use through an API. This is why there are quite a few services that offer an seo api through which you could be getting access to multiple tools that would be helpful for your SEO work.