Are there any good reasons for keeping date and time in separate columns?
My guess is that it's a legacy support thing (i.e back in version 1 of their software, they had date and time in seperate fields and they've been forced to support it because it always worked that way). 

That said, there's nothing brilliant about seperating date and time, so there's nothing you're missing.

(The other option is that your ERP system developers didn't know that a datetime field can store a date and a time, but far more likely is that they had a business requirement to make it work the same as prior versions.)