Even Free2Play games need to make money to be sustainable. Many have some sort of paywall, reserving certain content for people who pay. World of Tanks doesn't have such a wall, everybody has access to all the maps and tech tree tanks in the game. Instead WoT has something I'd rather describe as a payslope: The high-level game becomes very tedious if you don't pay.
The effect is first noticeable around level 7 (out of 10): Starting from this level regular tanks on regular accounts tend to spend more credits on ammo and repair than they get as credit reward at the end of the battle. There are some variables there, winning earns you more than losing, and dealing a lot of damage also earns you more. But with level 8+ tanks costing millions of credits, money definitely is getting tight at the higher levels.
People who want to play for free can farm credits with lower level tanks, which have a better revenue to cost ratio. But the general idea is obviously that if you play World of Tanks so much that you got into the high-level game, you should pay something for that privilege. There are two major ways how to increase your income of credits by spending real money: Gold tanks and a subscription.
When the level 8 gold tanks were added to the game, many assumed that these were mostly for the kind of players who wouldn't want to level that high with regular tanks. But by now many high-level players use such a gold tank to farm credits. All gold tanks are cheap to repair, and with credits earned depending on damage, the income from the higher level gold tanks is better. The gold tanks also appear to get more credits than other tanks for the same result.
Subscriptions are not paid for directly, but with the "gold" currency you buy for real money. At the most favorable rate, €100 or $125 buys you 30,000 gold, and 30 days of subscription cost 2,500 gold, thus the €100 or $125 buys you a subscription for a year. With a subscription you make 50% more credits and experience than with a regular account. You also get a nicer looking garage, but who cares? The 50% more credits shouldn't be underestimated, as this is applied to the credits you earn in battle before deducting the cost of repairs and ammo. So if with the regular account you made 8,000 credits, but paid 10,000 for repairs and ammo, you made a 2,000 credit loss. The 50% bonus increases your income to 12,000, and suddenly you made 2,000 credits profit. Thus effectively the subscription has a higher impact on your credits than on your experience, although of course the 50% bonus there are nice to have as well.
Theoretically you can exchange gold for credits directly, but I wouldn't recommend that. I already earned more credits with my Löwe gold tank than if I had exchanged the gold that tank cost directly for credits. Thus now that I have level 7 regular tanks and am close to the first level 8, I alternate driving regular tanks and the Löwe, so I'll have enough credits to actually buy that level 8 tank. At €25 the Löwe certainly wasn't cheap, but that one-time payment would be enough to get me over the payslope. I have a subscription on top of that, but that's just me spoiling myself. So if you want to buy only one thing, a level 8 gold tank is probably the best investment. You can get up that payslope without paying anything, but that would really tedious, with many low-level battles to farm credits. I like this business model, because it the pressure to pay something is more on the people who play the most. I find that more fair.