One defect in a sound roof
A slipped slate, cracked tile or failed flashing can often be repaired on its own. If the rest of the roof is straight, well fixed and dry underneath, there is little sense in creating a larger job for the sake of it.
Repeated repairs across the roof
Several old patches in different areas tell you the roof has a broader maintenance history. That still does not automatically mean renewal, but the cost of continuing piecemeal work should be compared with a more complete job.
Condition underneath
Battens, underlay and decking matter. A roof covering may look acceptable while the supporting layers are badly deteriorated. Conversely, an untidy-looking older slate roof may have plenty of life if the structure and fixings are sound.
Ask for the reason, not just the recommendation
If replacement is proposed, ask what specific condition makes repair poor value. You should be able to understand the answer without needing roofing jargon.