Can I put my phone on low mode when phone charging

Putting it in low power mode is useful if you will be unable to charge it for a long period of time. The downside is that not background processes will run, so it won’t sync iCloud or back up to iCloud, won’t automatically download app or iOS updates (won’t update iOS at all) won’t update email until you open the app, etc. As it severely restricts the use of your iPhone don’t use low power mode routinely, only when you are in a position where you can’t charge it for a long period of time.