If you're just plugging into a normal computer sound card, that's the reason why it doesn't sound as good as you'd like. Low-end preamps and converters. The reason for a balanced signal is largely noise - unbalanced lines (Such as an instrument cable) pick up a lot of noise over long runs, and balanced signals cancel much of the noise out.
Also, a reason for a direct box is that the output from an electric instrument like a bass or guitar is a high impedence source, and a direct box will convert it to a low impedence level, basically a mic level. So you can then plug it into a mixer or whatnot, and it will basically be the same level signal as a mic would be.
If you're just plugging into a stock sound card, a DI box isn't what you want to improve sound, you'll need a sound card or interface more dedicated to recording (THere's pci stuff such as the m-audio audiophile 2496 [which you will need a preamp such as the audiobuddy or dmp3 for, as it's just line level inputs], and there's all sorts of firewire, usb [although many people are wary of usb as a means of audio interface.... I use a usb interface with no problems, but many have had bad luck with them])
