Sous vide cooker and beer cooler
People do beer cooler cooks with a sous vide circulator (in fact, its noted by Anova, one of the most common sous vide tool manufacturers) -- primarily for going past the ~20 liter specification of the circulator to say 40-60 liters (e.g. by cutting a hole in the top of a cooler lid and sticking the circulator down in it; spray some insulation in the hole as well). 

For a small container (e.g. a stock pot, dutch oven) at room temperature, the common circulators have more than enough power to heat and circulate the water. But, you could wrap the thing with towels (as in the Anova link above) or something to help prevent heat loss. This can help with reaching higher temps too if you're losing a lot of heat. But yes, the circulator would have to work less hard at heating if you prevented evaporation loss and insulated the container. 

I normally do meat/eggs, and in my stockpot which I use for sous vide, my Anova (the current bluetooth model) doesn't have a problem maintaining temps. So, I don't use any towels or anything, just because its one more thing to deal with. I suspect most people use dutch ovens or stockpots or cambro containers just for convenience, and also get fine performance from the circulator.