Often is it hard to justify whether you completely own the IP. For example, you could've used the company resource to respond to an email related to your person

Often is it hard to justify whether you completely own the IP. For example, you could've used the company resource to respond to an email related to your personal project. Or some ideas for your personal project might've been influenced by your day work. Rather than delaying this type of problems till later stage, it is easier to just resolve such issues explicitly.