You don't need the pets if you go full Free2Play mode just trying to get a quick buck or if you create a bunch of alt accounts just to get some extra crypton for your main account. Pets are super useful in the game if you do want to progress toward bigger earnings and a point where you are able to exports NFTs to the blockchain. The more unique pets you own, the higher your odds will be to get more epic/legendary/shiny pets and collecting/buying them in the auction house allows you to make much faster account progress which has plenty of benefits.
In the Auction House, you can see which pets you own, have duplicates of, or are still missing (it has a number and a bar under them). I would say sell the duplicate ones (Epic/colored/shiny/legendary) and use that Crypton to buy ones you don't own yet that are cheap. Once you are level 25 you can buy 5 insanity deals (they will be nerfed for sure at some point) that will give 5 Sale points each level. with Pets you can increase your account level bar like crazy. -> See Twitch Clip
I'll write in greater detains on these things in the next post.