But the watching the altimeter like a hawk is a bad habit to get into.
Of course. But the idea is to get used to the forces necessary to hold pitch at various speeds, trim settings and power settings. If you do this exercise while looking out the window and you glance in at the altimeter, you'll find yourself being 100s of feet off altitude. And you spend the rest of the time recovering.
So initially you will want to keep a close eye on the altimeter so that deviations from your altitude don't happen, with the aim of being able to do this whole exercise, eventually, with only short glances at the altimeter.
Which is the whole objective in the end: To know and feel what the aircraft is doing and to be able to correct for this, to an extent subconsciously. While being able to look outside for other traffic and only glancing at the instruments every now and then to confirm everything is allright.
Ideally you would want to do this with a safety pilot on board of course, but that is not allowed while you're still at the solo stage.