Patreon is a form of "gift" you're giving the translator for the hard work. That itself is also a incentive since some trans patreons earn a bonus of 2-5k a month and those are the 3S grade with a great book and translation quality, Also those are the ones that are devoted to their translations IE, Ancient Strengthening Technique, Desolate Era, I Shall Seal The Heavens.
Where as Premium i believe is just a push for quantity and not quality. Honestly unless you're a full time translator with years of Chinese study or born Chinese and doing the translation. Anything more than 10-12 a week burns through quality like a son of a bitch. Think of Martial Arts Master, Though i don't know if it's just that specific translator but the earlier on chapters were translated in a very hard way to understand and made it tedious to read. Many times it made me want to throw my phone or smash my keyboard because of how frustrating it is.
Now back to the main point. Patreon is something you have the OPTION to support. For premium chapters you HAVE to support. See the difference?