For example, if a user is logged in, you can autofill the appropriate accounts/domains/ids/etc to make the example work out of the box; and if some ID needs to be essentially random, then you can make it actually random when you generate the example.
From a simple static page, now you need an API service, most probably connected to a DB or somehow integrated to the rest of your backend. So markdown suddenly isn't enough and you need some server-side logic.
For random strings, you can do this with client-side logic, which in some cases might be easier than server-side logic. But you are still moving from no-logic (static) to somewhere-logic.
as a develoer, is just a nice touch that I can copy paste an example of code snippet and since I'm logged in they can give it to me already with a valid api key.
ymmv
They also allow to output examples in as many langugages/sdk's as you need too