Why? Simply to turn off registration during certain hours (option 1) or to switch the forum to read only mode (option 2). Reasons:
- One of my forums is for children and the moderators have to closely monitor the forum not only for spammers but also for adults in disguise, often registering at hours when children are at sleep.
- At my largest forum we had one month where trolls where hitting us hard, they where extremely persistent, they knew our mods where pretty good at banning them during normal hours (7 am to 11 pm) but we all need some sleep from time to time
- Corporate forums which simply don't like to clean up a forum from nightly spam