In a nutshell Fifa Coins are a virtual currency for a game add-on withinfifa 15 coins(Created by EA Sports) called FIFA Ultimate Team, they are used as barter for buying players.
FIFA is a multi-platform game playable on a range of consoles such as PS3’s, Wii’s, PC’s, PSP, Nintendo 3DS, iOS and finally Xbox 360’s. However, FUT (FIFA Ultimate Team) was only introduced into the FIFA series in FIFA 09 as a downloadable add-on but nobody knew how successful it would be, not even EA. I would believe there are now more online games played on Ultimate Team than there are played on the original ‘Head to Head’ online game mode for FIFA that has been played ever since the first FIFA made in 1994 – it is that big.
So why does it help by having a lot of FUT Coins?

Ultimately the more coins you own the higher quality of football players you can buy and play with, this means your chances of winning tournaments will increase because your team will be superior in quality to your opponents. Tournaments are important because by winning them they reward the winner with bonus coins or sometime even bonus player packs to help you improve your team/squad giving you an advantage to the less fortunate.