It's easier to replace a cog than a chainring. Also, to obtain the same change in gearing ratio, you would need to have more difference in chainring teeth than sprocket teeth. This difference is relevant if, for example, increasing the sprocket by 1 tooth is all you need. Then you might get away without having to break the chain and attach a link.

EDIT: to clarify the last statement: say you have 46/16. You want to gear down for the winter and you would be satisfied with

46/17

or

43/16

They both give about the same gearing, but one will result in just one tooth difference, most probably allowing you to not touch the chain. The other requires 3 teeth of difference, and your dropouts probably won't have enough room to accomodate the extra chain.