Well, there's a few different kinds of light up vibes. There are the ones where there are lights on the controls, and the color/pattern varies with each vibe that does it. The purpose for some of these is so that you can easily see which setting you're on without having to guess. These are the more popular lighting options, but there are some (I believe) that just sort of light up because they can. Most of them with the vibration pattern I think, but since I don't personally own any that don't seem to have a reason for lighting up, I can't really speak for them.

They kind of bug me because they're not discreet. I often masturbate after my girlfriend falls asleep, and I don't want a bunch of lights flashing too wake her up. The blankets can muffle the sound, but sometimes lights show through the blankets.

The ones I've had (which are unavailable on EF) have red flashing lights. I don't know if they're the seizure-inducing kind as I am not epileptic.

Some of the Fun Factory vibes light up when they are on. Only the buttons light up. This is good, because then you can find the buttons in the dark.

I have never had a vibe with "flashing" lights. The Fun Factory ones are very discreet.

I thought they only lite up because it was like Christmas and you have a present to open. I'm bad....

Light can also generate heat which appeals to some.

i think is just for the looks