Rarely do I tab from sheet music. Normally, I just sit down with my harmonica, and start playing the song I want to tab. Once I have the song sounding the way I want it, I play the song very slowly, paying close attention to what hole I'm blowing or drawing through, and write it down. I do this phrase by phrase with the song, thus the words. If it's a song that doesn't have words, (or I don't know them) then I try to break it up into note phrases that make sense in my mind. After I'm done writing the tab out, I'll play it back exactly as I wrote it to make sure I didn't make any mistakes. I correct then correct any mistakes I find and post the song on the web. I then will go back and play it again from the web site to make sure I didn't mistype anything. I still occasionally find errors. In fact, I just fixed some today on songs I posted over the weekend.
Now for a more logical reason to have the words. Aside from making tabbing easier (at least for me), they also tend to trigger memories in people about the tempo and rhythm of the song. Some people might not quite remember the name of the song, but if they remember the lyrics and can think of the person singing the song, then they can usually pick up the beat. This is especially helpful if the song is not quite familiar, but you are fairly certain you've heard it before. You start to play the notes with a steady beat. The notes by themselves might not be enough to get the beat right, but with the lyrics in front of you as well, it can jog your memory and then the light will come on, and "DING!" you can play the song "properly."