Why use water bladders? For me it’s convenience. I find that when carrying bottles, I don’t drink as often as I probably should as I need to stop to grab the bottles out of my pack, unless I have side pockets. One thing I do find with bladders though it that I tend to take smaller sips and can’t always tell how much I’ve drank until I check the bladder, usually at the end of my hike.
If I’m only going for a short hike, I tend to take bottles but if it’s a long day hike, overnighter or multi-day, I appreciate being able to carry 3 litres in the bladder. I do always have a reserve bottle or container just in case I get a puncture though.