Changed Hot Water Tank's Hot Water-Line Valve On Top of Heater

About 2 weeks ago, I called a local plumber to change a corroded valve (atop my hot water heater) on a pipe leading hot water to the house.  For some reason, the plumber drained my hot water heater.....something I have never done for any hot water heater.

The only reason I asked the plumber to change the corroded valve was to be proactive so that the valve would not fall off & flood my basement one day.

Since the plumber drained my hot water heater  & changed the corroded valve, now, I am noticing water puddles at the base of the copper drain pipe leading from the "safety pressure valve" on top of my water heater.

I never had problems with this hot water heater before the plumber drained it, even though this tank is about 12 years old.  Always, I kept the temperature on the hot water tank only on warm (as I did for the last Rheem 50 gal. tank I had for 18 years.....even then, I only changed that tank because I was changing the boiler....not because it was presenting any problems).

Three questions:

1.    Should the plumber have drained the entire hot water heater in order just to change the valve on the hot water pipe leading out of the heater to the house?

2.    How dangerous, now, is this situation...... with the "safety pressure valve" constantly relieving pressure by releasing water through the vertical, copper pipe adjacent to the heater?

3.  Can the water pressure be lowered inside the hot water heater?

I would appreciate any & all input that anyone can offer.

I am a female running a household & am not too knowledgeable about plumbing issues (just the basics I've learned from prior experiences with plumbing problems).   Thank you so much!