Can use low power mode all the time?can this be helpful to protect battery life?

Yes. I use it all the time.

It turns off certain things to save battery life.

For instance your photos won't sync to icloud(they do sync as soon as you're charging it and the battery gets to 80%) and you can not use background app refresh.

https://support.apple.com/en-us/HT205234 you can check this for more information.

Low-power mode similarly disables many useful features, including background processes' use of wireless communications. This limitation makes a smartphone less, well, smart. We recommend using these modes only when you must, rather than as regular battery-saving methods.

Check out this Apple Support link for information:

Use Low Power Mode to save battery life on your iPhone