I like to know that the programs I’m developing utilize resources as I expect or intent them to.
I might be bad at my job but it’s not uncommon for me to design something and discover it’s sucking back resources to some crazy degree. I don’t want to discover that in flame charts way after the fact. I’d rather see it in real time and diagnose the issue early on in the process.
Basic insight into what your device is doing, and which app is doing what.
Lets you build intuition about what is normal vs abnormal. And it's also essential for letting you perceive which apps are bad citizens (and good citizens). Try one of these tools out for a month and you catch all sorts of random things.
Funnily enough, the computer's fan used to perform part of this role. But that has increasingly gone away as laptops become increasingly silent.
Once I randomly opened Activity Monitor on my M1 Macbook only to find that vim was using 100% of a CPU core, and the timer said it had been in this state for days! It wasn't even in the foreground of any terminal tab. Just in a spinloop in the background doing who knows what. And it might have stayed like this for a year until I'm forced to reset the computer for an update.
Another example that you might find more compelling is when your computer's network jumps to 5MB/s. It should always be explainable.
Not using any menubar stat programs currently, but have done so for exactly this reason in the past. Even if nothing is misbehaving, they’re helpful for keeping a pulse on your machine and for getting a feel for which programs are heavy on resources for no good reason.
It used to be that noise from hard drive activity and fans spinning up were a pretty good proxy for this, like how my iMac G5 would do its best impression of a jet engine whenever a flash ad banner appeared on screen. These days on M-series Macs though a random browser tab can be keeping a whole core pegged in the background and the only reason I’d notice is if I happened to touch the bottom of the laptop and notice it’s slightly warm.
Why?