That all said,
You can also naturally (and you probably should) use them on Crafting jobs for Tradeskill Leves, which means you need to make item X or Y for a client, who will reward you with gil, an item which usually are free crystals and lets not forget, a BOATLOAD chunk of EXP. More than you'd make crafting for a while. Definitely more than the measly amount you'd get from a combat-leve, you have FATES for your Deciples of War and Magic Classes, dont waste your points on it.
Same goes for the Field Leves, which are used by Gathering classes. Yield more EXP than you'd get just gathering normally. Big chunks at a time, which will make leveling trivial.
You should look at what best suits you for time versus reward. Asuming a relatively low (up to level 30 level)
Combat classes: 2000 to 7000 EXP per leve, but they can also do FATES which give the same or more EXP for trivial work and time invested. Please dont spend your Leve allowances on something that has alternatives that are easier, quicker, give more EXP and can be found all over the world in a matter of seconds. The one exception to this rule is ofcourse if your Leve quest reward is an item you could really need for your job.
Crafting classes: 4000 to 16000 EXP per leve, when you turn in HQ items instead, the reward gets an extra 200% on top of that.
Meaning 12.000 to 48.000 EXP per leve turn in. Not counting the leves which let you turn in items 3 times for 3 times the reward.
Which, depending on your level, is about 2-3 Leves needed per level.
Gathering classes: 4000 to 10000 EXP per leve, you do get higher rewards if you are quick and do them at a higher level, but rarely anything over a 25% increase.
[XI] Surivere of Valefor
[XIV] Sir Surian Bedivere of Behemoth