It helps me to map out what I hear. Sometimes I see the tab (especially from downloads), and although it doesn't sound quite right, I cannot figure out how it should go. This helps me to play around with the notes until it sounds right. It also gives me acurate timings, which online tabs almost never do.
Last, I can transpose to get a song into either a more comfortable key or (more likely) into the correct key to play along with a CD.
Although it is a bit pricey, I feel it is worth it.
What are you waiting for?