Sure, you can write on a computer. I hear that many writers do so. In fact, I recently read Lawerence Block's chapter in Spider, Spin Me a Web in which he said that writing on a screen was the best way to write because that way you have control over what the prose looks like. After many poetry classes where form and line structure were discussed, I can even see his point.
But very, very few things beat the satisfaction of taking a notebook that was once filled with nothing but blank lines...and filling it up. Page after page littered with words that didn't exist before I took a hand to paper. It's not as fun as typing 'The End' but there is a definite sense of accomplishment. You can point to that notebook at the end of the day and say "I filled that."
It's a definite motivation when you're in the middle of a long piece to point at the benchmarks. You can guage your length, your accomplishment, and know that something in the midst of all those word count goals became tangible. A computer screen is a computer screen is a computer screen. You can go on forever and not realize how much you've done utnil you hit the print key.
My way is great for those who are interested in immediate gratification.