If you run a small business then it's advisable to have a Twitter account. For a very low amount of marketing effort, you can catch the eye of consumers that you wouldn't otherwise reach, and if your twitter feed is clearly targeted to a certain market, you will pick up followers who are interested in exactly what you offer, creating a tailored sales channel.
On the other hand, if you are an ambitious professional. It also makes sense to have a personal Twitter account, allowing you to build your brand as a thought leader in your field. How to manage both of them?
The first service that Hootsuite or similar sites, such as Tweetdeck, offer is a simple one, but probably the one that adds the largest value. Without a place to access more than one feed you would need to log out and in again every time you wanted to look at a different account. With Hootsuite you can stay logged into your main Hootsuite account and then access all feeds, in a simlar way to the Google mutiple sign-in feature for emails.
Then there are services that are invaluable if you are a busy business tweeter rather than a casual personal one. You can send the same tweet from mutiple accounts. You can schedule tweets so that they appear in your timeline at a later date (perhaps while you are busy in a meeting!). You can even auto schedule tweets to appear when they will have the maximum impact on your followers.
Hootsuite is free to use if you want to access fewer than 5 twitter accounts, which should be enough to be getting on with for most people.