WiiHD

Please clarify what happens when I build a settler in an existing city and then settler it there rather than find a new place on the board for starting a new city. Is this advantageous at all?
Thanks.

A settler costs you two population from the originating city to make (unless you have a Republic for government), in addition to the production costs. When you add a settler to a city, rather than settling it somewhere, you gain one population in that city.

Not only would that, at best (if you have Republic) be a waste of 20 odd production and a wash in terms of population, there's the opportunity cost in that if you settle it it's adds at least 2 population to the new city (more as you progress technologically, or if you have a civ with a starting pop bonus). Sometimes, it might make a little sense to build units in a fast growing city while you're a republic and move them to a different slower growing (or non-growing) city, but not often.