If you have more than one of the same spell attuned, then it will allow you to cast it more before having to rest. So basically double the firepower at the cost of an attunement slot.

I currently have 4 slots, two of them fireballs, one fire orb and one iron flesh. Having 16 fireballs instead of 8 has helped me out A LOT, so I would highly reccommend it.

I would say it is very worth buying it, it will stack with the other in amount so instead of 8 fireballs u will have 16 fireballs, power is the same u just get more at the cost of using another magic slot