There are two reasons.
First, if you try to dig a well when you’re thirsty, it’s too late. If you suddenly spring into personal branding action when looking for a job, prospective employers/clients see that immediately. You serve yourself well if you build it over time.
Second, a powerful personal brand will yield results NOW, even within your current company. Think raise, promotion, new client, new opportunities. When you polish your online presence, it not only benefits you, but also your company, and your employer knows that.