1.25kgs are almost always the smallest a gym will have, which is fine for large number of men and how most people in gyms train (rep ranges), but if you're on the lighter side of things as a male or for most women generally, 2.5kg jumps on a barbell for upper body stuff end up being too great if the goal is straightforward 'add weight every session, do same reps' and/or mean you'll have to start doing something different quicker because 2.5kg jumps are easier for someone weighing 90kg than someone who is 60kg (or a little 50kg laydeh). The old 1kg PB can be a useful way to eek out linear progression for longer before resorting to periodisation, if set numbers are your thing.