I guess depending on where you are.
If you are in Singapore, their tap water is approved by health authority for drinking straight out of the tap. As a practice, we usually boil them before drinking at home. Back in university, we can't be bothered with boiling the water, and we just drink it straight out of the tap.
In Hong Kong, we have a water filter and hence, drink water straight from the water filter without boiling.
I suppose if you are in country with less proper water sanitation, it would be better to buy bottled water.
For myself, when travelling, I will buy bottled water because you really don't want to have issue with the water which can spoil your vacation plan.