Okay, so... They each are for an individual element, obviously.
• Bombos - Fire
• Ether - Wind
• Quake - Earth
But, they have their different uses... I actually use Ether the most when I am just playing around because of boredom. I find it amusing to freeze my foes with it--even though I have an Ice Rod which can do that, too... But the best part is picking frozen enemies up and throwing them... and then you can even throw them at other enemies--and cuccos, if I feel like risking that and am in a spot with them nearby such as Kakariko Village. Ether, though, can actually be useful in caves/rooms with invisible tiles, so that you know where they are--but I would usually use the Cane of Somaria for that and just push the brick until it falls.
I think the only reason I like Quake is because I find it epic how Link will jump up and land with his sword planted into the ground, and then that causes the ground to tremor. I like earthly spells a lot, but... eh, earthquakes. It is exciting at first, I guess, but I don't really use the spells that much in my primary file since I've already beaten the game (my first file is a keeper--I won't erase it!). So, if I'm not using it when I'm just messing around, it just sits in Links pockets looking pretty and no earthquakes.
Bombos is awesome because its element is FIRE. Fire is awesome. I mean, really... Fire balls orbit Link, then break out into fiery explosions, if I remember this spell correctly. Again, I don't use it as much as Ether when I'm playing around, but I do think I may have used it in Turtle for something... :/ Oh, but this spell is amusing because the enemies that are in the same room as me when I use it, they momentarily freeze motion, catch on fire, then vanish. It's even more amusing if I use it after I already have frozen them with Ether or the Ice Rod.
So, Bombos and Quake are neat--don't get me wrong. But, they're a little too... torturous for the poor dumb enemies that live in A Link to the Past. Their AI cannot comprehend what is going on. If I'm just playing around, I'm kind of just killing things for fun... because... well, boredom. At least if I use Ether, they just freeze and then don't have to hurt their brains trying to figure stuff out while I do anything else mean to them.