I have already mentioned, an agent goes by territory. Those covering Sentosa, they don't cover Serangoon Garden, for example. They can, but they don't.
The landed property agents, they are not the typical condo agents. They possess a great deal of knowledge of the areas they are covering. They can tell you about the history of an estate, value of a land or house, things about rebuilding, the better contractors, architects, financing etc.
A landed (whether for a old one-storey, or a newly rebuild), in good location, with reasonable asking price, it didn't get advertised before being sold. Once the landed is put on the market, the agent will know, even when she is not the agent appointed by the selling owner.
(Just check the URA caveat for landed transactions, how many of them were actually advertised?)
If you were to buy 2 units of condo, you have a ready resource of $3 mil, then you want to be certain that you are in the vvip lists of the developments you are interested in. To be able to get the choice units at lower prices.
Especially so for landed. Along one road, one Jalan, how many units there? 20, 30 units. How many old single-storey units? So, unless you retain an agent(s) to scout, the good ones, at good prices will be gone before you
Those appeared in advertisements, Strait Times classified, Propertyguru, etc, are leftovers, those already 'rejected' by the agent clients.