iPad is my primary multimedia device at home to watch, read and research.
My MacBook is always on my desk, connected to the second monitor. I prefer to avoid constantly plugging and unplugging these connections (DisplayPort, Charge, USB-C Hub etc.).
Of course, I take my MacBook with me when I need to work outside. (Or when I need to use more powerful or complex apps at home.) But when I'm tired of sitting at the desk, I spend time with the iPad in bed.
iPhone is small for reading and watching, and its battery life's poor than the iPad. MacBook is huge for a bed. And, sometimes, I can fall asleep watching something. (Dangerous for expensive gadgets like a MacBook and iPhone.)
Also, the apps I use on my digital piano are incompatible with the MacBook or iPhone.
iPhone cannot be connected to the piano via USB-C. Even if connected, the screen is small. MacBook does not support the app. Even if supported, the size is large for placing on the piano.