The main benefit of premium is an increased monthly character limit, by default, you get 6000 characters per month. This can be increased all the way up to 5000000 if that is so required!
You get a few other benefits such as access to Google Translate, Deepl Translation, React to Translate, and increased Wordcloud limits.
What Tier? There are so many!
At first glance, it may look like there are 24 separate tiers, and whilst there technically are. In practise it's 8 tiers, with each tier having 1, 3, and 6 server options.
Which tier you need depends entirely on how many characters you need per month. If you run the /translation-usage command, this will show you a monthly prediction at the bottom, although be aware it isn't always super accurate, especially not early in the month.
The best thing to do is start with what you think will be enough, as you can always upgrade later on and you only have to pay the difference.