1. Translates to other instruments. Learning to read music enables faster learning on other instruments if you ever decide to learn another one. Listing this one first for me as I started on French horn. Learning bass was significantly easier since I could already read music.
2. Access to additional music. Not everything is available in tab.
3. Faster learning on new music/ability to sight read.
The benefit that I rarely see mentioned is what I call rhythmic awareness.
It's knowing precisely* how a rhythm goes, vs playing what you sort of remember.
It is especially powerful for a bassist.
*of course there will still be room for interpretation, but you have a precise model to vary.