When you want to rank on Google: Google ranks individual pages for specific keywords. A one-page website can rank for 1-2 keywords at most. A multi-page site with a dedicated page for each service, each city, and a blog - ranks for hundreds of keywords.
Example: A coaching center on one page can rank for "coaching center in Bhopal." A multi-page site can rank for "NEET coaching Bhopal," "JEE coaching Bhopal," "spoken English classes Bhopal," and 30+ more individually.
When you offer multiple services: If you serve doctors AND salons AND coaching centers, one page can't tell Google's algorithm that you're relevant for all three. Separate service pages solve this.
When credibility requires depth: High-value purchases (a INR 1 lakh website, a INR 50,000 wedding photographer, a INR 20,000 coaching program) require the buyer to build trust before committing. Case studies, detailed about pages, multiple testimonials - these need space that one page doesn't provide.