Harmful to never put to sleep / shut down a mac?
You can safely leave your Mac powered on all the time.

Safe and Harmless

There is no practical damage or harm that will occur because the Mac is constantly running. Use your Mac as best suits your needs.

Restarting is helpful every so often – if only to install updates and clear out caches or temporary files.

Environment

As 1lann mentions, environmental factors like dust and heat build-up should be considered. Keep your Mac away from sources of dust, like limestone or sandstone walls, where the fans can draw in surprising amounts of particulate matter.