This is what happens when you replace a slate roof covering with concrete tiles which can be three times heavier and don’t bother to strengthen the supporting frame: it sags and distorts.
In this instance the previous home owner or their roofer/builder realised a bit late in the day what was happening and provided some after the event strengthening: a timber strut jointed to the underside of the horizontal purlin which also has a steel reinforcing plate bolted to it’s underside.
The arrangement is acceptable and means the roof is unlikely to move further. However, from the outside the dip in the roof slope does not look particularly good. At a future date complete replacement of the roof frame and covering will have to be considered.