It is a way they have the meeting in their calendars, get reminders and don't forget to join you by that time.
In Microsoft Teams, I can arrange a meeting as fast as I can set up a calendar event in Outlook, so why not have both issues covered at once?
Moreover, Microsoft Teams has features for meetings such as call recording, that are not available in 1:1 calls. Finally, it is easier to onboard somebody else in the call, if needed, by just sending them the meeting link or adding them.
Then, the planning and organization side, where you positively occupy a slot in your calendar so there is no chance of getting into something else by mistake.
Calls are meant for inmediate, likely short conversations. If it is something that must happen at a specific date and time, it may be because it takes a while (30-60 minutes...?), in such a situation, it is better to just arrange a formal meeting.