Two systems talking directly is simple right up until it is three systems, then five. Where the line actually sits.
Count the edges
Point-to-point between n systems tends toward n² connections, each with its own auth, retries and error handling. Middleware turns that into n connections to one hub.
But do not buy a bus for one route
One Salesforce org calling one API does not need an integration platform. Start point-to-point behind a named credential; move to middleware when the third system shows up.
Practice in your org
- Open your Developer Edition org and follow along step by step
- Change one thing at a time and note what happens
- When something breaks, read the error before searching for it