Best example to answer this is to scribble something complex on paper with a pen, scan it and open this scanned image in photoshop. Now ry copying that scribble in photoshop with your mouse. Unless your hand is extremely steady, it's going to be hard to copy that shape exactly. Now imagine doing that with a tablet, (taking in mind that with a tablet you are using a pen and when writing on paper you're using a (you guesed it) pen. you can imagine how copying that complex scribble will be a whole lot more accurate.

Plus with a tablet you get added bonuses such as "pressure sensitivity" which is good for things such as hair for example.

Hope that helps, giving you a practicle example.

Dell XPS M1730
Intel Core 2 Duo T9500 2.6Ghz
500GB (2x 250GB) RAID 1 Sata HDD
4GB RAM
Nvidia GeForce 9800M GTX SLI