With all the online transactions you’re making and all the spam, phishing, and malware that’s out there, using multiple email addresses to separate your email into different buckets can help keep things secure, or at least easier to manage.
Work
Use your work email address only for work. First of all, it’s company property, which means you have no privacy whatsoever. Your messages may be used if the company gets involved in some kind of legal matter or is looking for reasons to fire you with cause. Plus, you can only access your messages while you remain employed by the company. If you do leave your job, you’ll run into a big hassle when you try to make account changes on a lot of websites (which often confirm the changes by emailing your primary email address).
Money
Use a separate email address for important things like your bank, credit cards, cell phone plans, and utilities. These are the kinds of things where someone with access could take money from you, run up some fraudulent charges, or affect your credit score. Separation protects you from yourself because it’s much less likely that a separate email address will get phishing emails claiming to be from your bank, which means you won’t install malware or enter your credentials on a phishing site. If this address somehow does end up on a spam list, it’s relatively easy to set up another account and update your account details on a limited number of sites, as opposed to every single site you’ve ever signed up for.
Purchases
I would separate purchases from the banks and utilities since this address will be more widely distributed and the level of security on commerce sites is uneven. Amazon is secure, Pa’s Widget Shop is only probably secure. Worse yet, some of the sketchier commerce sites might rent or sell their email lists, especially if they get into financial trouble. Some of the poker and gaming sites definitely fall into this category: most casino sites, defunct Party affiliates, and third-tier poker sites that have no hope of being as successful as Party, Stars, or Full Tilt.
Personal
This is the email address you use for all your correspondence with real people. When you get an indication that you have new mail for this address, your brain should get a double shot of neuro-chemicals and become very excited. Separation here protects you from yourself and your friends. Unfortunately, your family and friends will give your email address to a spammer unwittingly at some point in their lives. Fortunately, you’re not expecting non-personal emails at this address and can quickly identify what is spam and flag it.
Other
Use a fifth email address for everything else. As time goes by, you might see a category of websites that merit a separate email address (e.g., poker sites), but for now, all the non-critical stuff can go here. This is the account that you use to sign up for anything and everything, so it’s definitely going to get slammed with crap. But that’s ok.
For those of you who are uber-paranoid, OCD, or otherwise troubled, you can take all this a magnitude higher by registering your own domain and setting up a separate email address for every website you interact with.