[jiveturkey]

For pasting of passwords, there is no security benefit. password managers ignore it.

"They" often also disable pasting of other duplicated fields, like bank routing number, email address. So this shows why it is done (but misapplied to password). It's so you don't just copy and paste a wrong value that is hard for them to verify and leads to support calls. By forcing you to retype from scratch, the theory is you will either get it right twice or the error will be flagged.

[megaremote]

This is so silly, I am more likely to get it wrong if I type it in than paste it in.

[jiveturkey]

but you aren't likely to type the same, matching error twice.