No offense, but that’s not really much of an answer beyond “it’s that way just because”. “Interval” suggests a regular pattern, while the Weekly function is, in every single other app I’ve ever used, intended for custom distributions by specific weekday. Is there some sort of boundary problem with the code that it can’t handle an iteration of 1? Or did the developers simply overlook that fundamental use case? Those things happen, no big deal — but there are actual solutions.
There is no technical barrier — the interval can certainly be 1, there is no boundary problem. It’s just that the current way is what the firmware has been since beginning, and no one has ever requested the ability of settings interval to 1. I think people are used to the way traditional sprinkler controllers work where you set a program, and you choose weekdays the program runs on, and if you want it to run each day, then just select all weekdays. Back in my mind my reasoning was probably that ‘weekday’ schedule and ‘interval day’ schedule should cover different sets of capabilities, so that if one of them already supports ‘everyday’ capability, the other does not need to overlap and cover the same capability as well.