A relative expression (mawṣūl) is useful when a hearer does not know a person’s name but recognizes an event or characteristic connected with that person. “الَّذِي كَانَ مَعَنَا أَمْسِ” means the person who was with us yesterday.
الَّذِي كَانَ مَعَنَا أَمْسِ رَجُلٌ عَالِمٌ
The one who was with us yesterday is a learned man.
The clause “كَانَ مَعَنَا أَمْسِ” supplies identification; “رَجُلٌ عَالِمٌ” gives the predicate. This is not needless circumlocution when the relative clause is what allows the hearer to locate the intended individual.