Wargaming is a business, so the need to make money. One of the ways they earn their money is by releasing premium tanks you have to pay for (or buy using gold, which is the same thing).
Every now and then, some of them looks exiting, and you want to buy it. I would advice you to consider this before you buy one: Do I already have a premium tank of the same type (light/medium/heavy) from the same nation? If so: Are you going to play it because it seems good or because you need to train / retrain your crew?
If it is to (re-)train your crew you should focus on buying a premium tank of a different type and/or nation than you already have. This might seem obvious, but I have to admit I have spent far to much money on German TDs and Russian medium tanks. (I have six of the latter and five of the former.) Choose where you spend your money wisely!
wotnewbie 