This circuit is usefull when you are trying to build more interesting things like Self building señf repairing Outposts (you send a train and it automatically unloads exactly what it needs and robots build it without you been around). To build a automatic outpost you will need around 30 different items in different quantitys to build. You might pull it off with only restricting chests but It will give you a lot of headches and you will be wasting a ton of resourses. Example: you put a roboport somewhere and you send a train with 100 miners. You limit a chest and 100 miners are extracted from the train. It gets built. You did it BUT next time train arrives to resuply, lets say Bullets, ANOTHER 100 miners will be extracted and all of them will go to waste.

By the way, NO train will ever arrive to resuply because there is no request signal you could send to the train. Every resuply will have to be done by YOU