The iron sight on sniper rifles is just a backup sight. It should be fixed for a distance like 100-200m.
Some sniper rifles have an adjustable iron sight (like the G22) for 200-600m.
BTW:
The beloved M82 Barrett got an iron sight for backup as well. And it is also zeroed for 200m. (I`m not really sure about it, it's been a while since I get in touch with this weapon.)