Whenever I'm playing anything melodic on bass or guitar, I'm always bending the strings a little bit if I linger on a note for just a fraction of a second. something I like to do on regular guitar is bend up to a note, then release it a little, and then bend back up with vibrato. It really livens up a note. I think the aim is to disrupt the dead-on frequency caused by having frets. I'm not a whammy bar user, but players like Gilmour, Beck, and Holdsworth are/were masters at using that thing to subtly dip in and out of notes. It's like trying to dance around the note in an effort to replicate the way a natural human voice sings. This is why I always think violins and fretless basses always sound more expressive. Violins especially because they can also swell the volume of a note like a voice can. you can do this on guitar with the volume knob or a pedal, but it never sounds as natural. I have this half-baked theory that blues evolved as a result of guitars and pianos being tuned to equal temperament and playing notes dead on. The African Americans who developed that style were used to people singing harmonies like the major third really sweetly in tune, but equal temperamment instruments don't do that: the thirds are tuned pretty noticeably wide. That is the upper note is quite sharp of where it would be to make the interval totally smooth. On an equal tempered instrument, that sweet major third lies somewhere between the minor and major. Obviously a lot closer to the major because that's what it is. So blues players used bottlenecks and even knives to find that note that lay between the frets. If they weren't using a slide, they'd use bends. I'm sure this played a part in how blues evolved.