They are adapters between the same series, male to female, used to protect both the male and female connected to an important instrument, here are some examples: Let's say you have a reflectometric bridge up to 18 GHz with SMA female output, certainly you will regret when you’ll find that the wear of the connector has irreversibly damaged it, so it will not work properly anymore. The remedy is just to put a SMA male to female adapter in connection with the instrument SMA female connector to save it from the continuous usage. But in case of sophisticated and delicate measurements like return loss or the head of a bolometer, etc. it is better to invest more money in buying a good quality adapter because a standard SMA could bring some errors in measurements especially beyond a certain frequency. Only in these particular cases it would be better to use a good quality adapter to protect the measures and in particular cases you can always remove the adapter to do the measure.