Mining rewards are paid to the miner who discovers a solution to the puzzle first, and the probability that a participant will be the one to discover the solution is equal to the portion of the total mining power on the network. Participants with a small percentage of the mining power stand a very small chance of discovering the next block on their own.
For instance, a mining card that one could purchase for a couple thousand dollars would represent less than 0.001% of the network’s mining power. With such a small chance at finding the next block, it could be a long time before that miner finds a block, and the difficulty going up makes things even worse. The miner may never recoup their investment. The answer to this problem is mining pools.
Mining pools are operated by third parties and coordinate groups of miners. By working together in a pool and sharing the payouts amongst participants, miners can get a steady flow of bitcoin starting the day they activate their miner. Statistics on some of the mining pools can be seen on Blockchain.info.