How much and how do these help you? Does your reading vary a lot and does it correspond to your HR and/or BP?
It seems to help a fair bit in avoiding crashes and similar worsening of how I feel. If I avoid extra activities when my heart rate is high, such as in the 95 range, then I can avoid aggravating symptoms.
It can also help with determining how much of an effect an activity is having - for example, a bath can get my heart rate to 150 or higher, with it then staying at 115 for hours and 95 for days (this feels REALLY nasty - can't sleep when heart rate is that high). But if I'm very careful about pacing during my bath, I can keep it from getting as high, or staying elevated for as long.
So if I'm not careful, my heart rate hits 150 or higher and I feel very sick after a bath, sometimes for a long time afterward. But if I go upstairs, lay down til heart rate is 85, rinse the tub and start the water running, lay down again until heart rate is 85, get in the tub and stay sitting in my slouched forward position until heart rate is stable (usually 95-100 at this point), then wash my hair, then rest until it's stable again, then do the conditioner, then slouch again, then dry off upper body a bit, rest a bit more, then get out, dry off, and lay down immediately until heart rate is stable under 90. That might require an hour or more of resting when getting out of the bath.
Based on experience, I now know that f I can't get my heart rate down to 85 when resting, then I shouldn't be taking a bath. And if rush the bath, and go downstairs immediately instead of resting as soon as I get out of the bath, my heart rate will stay elevated for a very long time, with oxygen levels trying to dip (and sometimes succeeding).
So basically I'm using it to see how badly an activity is impacting me, and how successful alternative approaches are. The weak point thus far is me not using it at times and continuing to push myself to hard when I really want to do something
Another use I've put it to is seeing what happens when I lay down in different positions. For years I've felt like I can't breathe properly after half an hour or an hour on my right side or back, and the pulse oximeter confirms that my oxygen levels drop and/or pulse rises in those positions. Similarly, if I sit up leaning back slightly on the couch, I run into trouble, and do best if sitting slouched forward with my elbows resting on my legs.