Why is it better to enter time constraints rather than unavailability?

When you enter unavailability constraints, these apply precisely to the slots painted red on the timetable. When you configure time constraints, you define a set of rules to be complied with: there are probably several slots that correspond to all these rules.
During automatic placement, time constraints offer the software greater calculation flexibility. Use unavailability only when the teacher's presence is impossible (for example, if the teacher has classes at another school).