I'm not as far into investing/maximizing returns/tax advantages as a lot of these other people, but I like the freedom that a taxable account affords. I can do anything with the money in it. Now that I've learned a thing or two, I'll be maxing my TDAmeritrade ROTH every year(and can do so for several years off of my taxable account if need be) for the tax benefits of that, but knowing that I have a couple years of funds available before starting to worry about tax treatments and early withdrawal fees in my other accounts affords me a certain peace of mind. As others said, it can be viewed as an emergency fund for anything that should need it, as a buffer to absolutely anything that could happen.
Also, it has way more options because I have full control over it compared to the company 401k, which though it offers lots of investment choices, doesn't offer me the one thing I'd jump at: a Dividend Growth fund.