Finger pulse oximeters are mainly useful to reassure you that your oxygenation is OK (90+%), or that you should don oxygen or troubleshoot the situation (85% or below). The fact that their readings are often inaccurate and misleading does not mean that they are useless!
What is oximetry?
Oximetry is a procedure for measuring the concentration of oxygen in the blood. The test is used in the evaluation of various medical conditions that affect the function of the heart and lungs.