Materia...what exactly is it?

[Garlyle]

Regarding Materia, Is it important to have it or just a "bonus"?

I main a tank class so having the best gear I can is important, but I haven't found out how to use material unless maybe I level a craft? Is it necessary?

It's optional. Generally, the best gear in the game (everything level 100+) doesn't even have Materia slots, and the closest stuff that can be crafted that has slots (level 90) will cost you an arm and a leg and is questionable if it even gives that much of an advantage, although as mentioned, crafted accessories are a good way to add chunks of accuracy without sacrificing much in the way of stats.

There's some usefulness for it if you get into PvP - all the specific gear for the Wolves' Den wants to be worn over PvE gear generally due to Morale, and all the PvP gear has materia slots on it; on top of the stats, ailment resistance materia can also be incredibly useful in PvP (mainly Stun Resistance/Sleep Resistance), unlike the main game. This is mostly for Wolves' Den 50s though - Battlegrounds players will find this less important (though nothing necessarily wrong with bringing i70 accessories with stun/sleep resistance slapped on!)

There's two major cases where it's not optional:
1. Crafters will find that once they hit 50, there's very little gear upgrades past the class-specific l50 gear/l50 quest tool - all the extra Control/Craftsmanship/CP they want (and they do need a lot more to make 2-star and up recipes) has to come from melding Materia to that gear, all of which has slots for melding. This is the big one.
2. The Relic weapon quest early on requires a specific weapon with two specific grade-III materia melded onto it, which will eventually be transformed into your relic. Far later, you also have to meld about 75 points worth of stats onto an item in order to complete your Relic Novus. While I think many players are nowhere near even doing Relic Novus yet, many players that hit fifty do undertake at least the first phase or two of the Relic quest (because getting it to its Zenith form is relatively easy, quick, and gives you a weapon that currently will serve you through basically all the content in the game) so the first step is important. That said, you can always just buy the stuff and look for a capable crafted in your free company or via party finder to meld it together for you.