What is the API portal for on my magento account? I can see how to create a new API, but I cannot find anyway of connecting that with my Magento 2 installation?
And if I did what does it provide?
If you want to use API, open web URL/swagger.
You'll get all the API listed there and use it from there itself.
I am guessing you want to know why need of Magento API's, why we have already integrated in Magento.
If so, then when we require to build any mobile app or integrate your website with a third party website to manage your orders/customers/products then you will need it.
But without even using the above things Magento is already using it in some modules as it makes security high, you can call it Service Contracts .
You can find blogs why you need to use Service contracts and how you can create it.
Hope it helps !