Relay notifications or/and messages, and other useful functions, like automated control of your home server and other electronics like vacuum robot and such from places with bad connection, also automated fetch of information you need, like timesheet or news you follow or state of your home and this is just fraction of use cases the only limiter is your imagination
Cause mammary glands, you’ll never know where you pumping that synthol into when it needs to go to muscles but not in mammary glands, also there’s alot of veins and other canals where it doesn’t need to go, but with arms it’s whole another business, muscles are big and avoiding arteries and veins is much simpler