I would think they're mostly for aesthetic reasons.

Makes sense. No organs in there, just goo.

For me, the nose is just for decoration and the breath of a slime is done through its skin.

I love this answer!! Thank you for your participation. *Hands you a lollipop of questionable quality.*

Thanks for the lollipop and... If it's coming from you, side effects are a blessing! (Just keep my conscience, the rest is profit)

I believe it's psychological if they became a slime where they'll form nostrils but no longer breathe...but don't quote me on that...because my slime OCs were born slimes and therefore...does not have nostrils/breathes

https://www.furaffinity.net/view/48454348/

https://www.furaffinity.net/view/46663413/

If the slime is a former organic then the nostrils are likely trying to mimic the olfactory sense. There may also be a vestigial comfort in a remnant breathing instinct.

It's also a conformity when trying to socialize with organics, they expect to see nostrils and may be unsettled by a slime who lacks them.

I would think that if there is an oxygen requirement, it can probably be fulfilled by epidermal osmosis.

My energy is technically unlimited but comes in cycles that resemble the organic sleep and meal schedule I used to have.

Whether a slime requires oxygen would depend on whether they evolved in or were designed for a low-oxygen environment. Pretty much all organisms require chemical sustenance from their environment, but there are organisms (primarily some types of bacteria and fungi) that don't require oxygen for their metabolic processes.

As for whether a slime's nostrils are useful... well, they could primarily intake oxygen through their skin, and the nostril could be purely an aesthetic comfort thing, either for themselves or other lifeforms around them that possess nostrils. But, there's also the question of how slime creatures could communicate through speech! The most likely explanation would be that they intake air through an orifice and pass it through a system that mimics non-slime organisms' vocal cords, and the slime's nostrils could be connected to this system, either playing an important part or perhaps simply a vestigial characteristic.

These are some great real life explanations, your participation is much appreciated! *Offers you a severely dented can of baked beans, three quarters of its' contents have already leaked out.*

'Nuff said.

In the lore I use, slimes don't need to breathe (so they can go underwater and so on). But some slime critters can create nostrils/noses/snouts/etc. just to mimic other creatures, or to use it as a sensory organ and smell the tasty foods around. Slimes still need to eat tasty things!

I think they may breathe like single-celled organisms by diffusing air across their surface. But they could have lungs and other parts of the respiratory system to increase their surface area to breathe better.