For the serious and large scale investor, gold bars are a simple and efficient way to invest in gold. The larger bars are usually available at the lowest premiums over their intrinsic gold value, smaller bars tend to cost more. There is a trade-off however, in that larger bars are not as flexible when it comes to selling. If you own a kilo bar, and you wish to sell, say 100 grams, it's not easy to slice off one end of your bar. Your choice of buyer is also more restricted as you will need to sell to a larger dealer, it is unlikely that you will find a private buyer as most people are not familiar with gold bullion bars.
It is sensible to consider modern one ounce gold bullion coins as being one ounce circular bullion bars, guaranteed by a government rather than a refiner.
Because coins are mass produced, and very efficiently so, they are available at very competitive prices compared with similar size bars. Because gold coins are almost universally recognised, they are also easy to resell.