Mixed theories on that, and most are older.

On earlier computers, I had several ICs walk themselves out of sockets due to repeated thermal expansion cycles. Keeping the computer turned on eliminates most of that.

Mechanical wear was another problem. Booting a computer was extremely taxing on old HDDs and floppy drives.

Edit: Mechanical stuff also takes much more power to spin up and get running. The energy savings might be measurable if you just kept a computer running and didn't power cycle it everyday.

Most power supplies are really well designed now but they had a tendency to spike power briefly in when turned on. This was especially bad for older capacitors but also not healthy for the ICs. This still happens to a degree, but it's not an issue.

Now that boot times are reasonably fast and most everything is solid state and power managed really well, turning a computer off is fine.

However, I just assume most electronics now just go into some type of deep sleep mode unless fully disconnected from any power source. That likely isn't true in many cases, but I consider it healthy level of paranoia.