Reading acceleration on the Bangle.js

Accelerometer would measure zero in every axis if there was no force acting for any axis. When you dont move you have the gravity force pulling you down and accelerometer measures that (if you are not floating in space). Try applying any acceleration (fast movements) in any axis and you will see that too. Here you have app for banglejs plotting accelerometer so will be easier to see that https://kristosb.github.io/WebBangle/
If you want to sample fast, accelerometer will be better option than GPS. Accelerometers have different ranges. Some are better for measuring earths gravity (low range) other better for high forces like in an airbag. It depends whats your application.