Unlike a premium account buying a premium tank has no time limit and it will be there whenever you log on(unless if you sell it). Premium tanks earn more credits and experience compared to similar tanks in their tier. They have a small experience modifier that will net them more experience which might not sound fair but there is nothing to research on these tanks. The experience can only be used to train crews if you check off accelerated crew training or converting using gold into “free experience.” The credit boost is why most players own a premium tank and these boosts allow many to earn 1.5x-2x the amount of net credits per battle than similar tanks in that tier. Premium tanks from tier 2-5 make a decent amount of profit but the tier 6-8 premium tanks are where you will see a noticeable difference in income.
One other small bonus is the ability to use crews not trained for the premium tank with no penalty. Crews will gain crew experience while in the tank allowing you to grind credits. Only crews that are the same nation and tank type will work without the need of being retrained. For example the tier 8 American premium heavy tank T34 is capable of having any American heavy tank crew(like the T29 or T110E5) placed into it with no need of retraining. This crew can fight with no penalty while still racking up crew experience.