pH UP/DOWN alters the pH almost instantly, but the problem is they don't KEEP the pH at that level for any length of time ... so over the day, the pH goes up & down ... much more stressful than just letting the fish acclimatize to a new but stable pH.
To alter the pH (or to doctor the water in any way), you need to have a good understanding of buffers, buffering capacity, acid/base balance ... etc. 99% of the people don't need to do that. Much easier to stay with what you get out of the tap.