The number one reason I've seen is for things that need visibility during the work day. For example, if my girlfriend needs to email me something I need to take action on during the work day, she knows to email my work email. Same with people who are doing time sensitive things for me (e.g. my realtor during a recent home purchase). Sure, I could start monitoring my personal email just as closely as my work email, but it's preferable for me (and probably preferable for my company) that I use my work email for work & time sensitive personal items, and use my personal email only outside of work for non-time sensitive personal matters.