Actually there's no "extra" reason to learn scales if you read standard notation. The advantages of learning/practicing scales are the same whether you read tabs, notation, or nothing at all.
Those advantages include:
- Practicing scales helps you develop and maintain speed, dexterity, and fretboard knowledge. (You can play a lot of different exercises to do this, actually. Scales are simply the most common ones.) You don't really have to understand the scale to get this benefit--you just have to be able to play the "shape".
- Knowing scales and understanding how to apply them will help you with writing your own melodies, improvising your own solos, and picking out songs by ear.
- Scales are the backbone of music theory. Understanding scales is the first step toward understanding how chords are built, why they're called what they are, and what to call that weird chord you just made up.
- You'll also better understand the concept of "key" and why certain chords seem to always appear together in the same song.
- You'll be able to think of chords in terms of their relationship to the key's scale. This will help you analyze songs, memorize songs, play/write Nashville number charts, and figure out chords quickly by ear.
- Knowing scales makes it a lot easier to transpose a song from one key to another.
I'm sure I've forgotten about a few...
JJ