I think this question falls under "equipment", but if it's off-topic, please close and I'll delete it.
I recently acquired an Instant Pot. To my surprise, its "steam" setting generates and uses pressure. If I steam something for 10 minutes, for example, I have to manually release the pressure or let it equalize naturally before I can open the lid. This has deleterious effects on certain foods. For example, idlis will not puff up the way one would expect; the pressure prevents it.
I hadn't thought that steaming food used pressure, generally speaking. The Wikipedia page on steaming explicitly contrasts the two:
The food is kept separate from the boiling water but has direct contact with the steam, resulting in a moist texture to the food. This differs from double boiling, in which food is not directly exposed to steam, or pressure cooking, which uses a sealed vessel.
My earlier rice cooker (which the IP replaced) had a steam setting as well; on that one, steam escaped continually from the vent during the steaming process.
I'm trying to figure out what the advantage is of sealing the vessel and building up pressure if the intention is to steam food. The product manual is very sparse on detail regarding steaming, but does mention that steaming vegetables "require[s] 1~2 minutes pressure cooking time" and that pressure must be released manually after steaming.
Is it the case that the "steam" function of the IP is not actually meant for steaming, say for custards or dishes that rise? Is it meant for pressure cooking vegetables in a way that replicates the effect of steaming, but faster? I tried searching online to find out more about how the steaming function of the IP works, but found nothing useful.
Best Answer
TL;DR
Why?
Assuming it doesn't turn out to be user error (I don't own one to confirm), I believe the "steam" function is just intended for pressure steaming. After all, it is a pressure cooker. So you'd use it when you need to do pressure steaming.
It's possible that the button only says "steam" and not "pressure steam" because the designers felt the use of pressure was implied for this product. As for why it just says "steamer" in the product description, that's a question only their marketing department can answer. They could've gone 8-in-1 if they counted "air dry" (lid and electricity not required) or "funny hat".¹
Here is a long-winded YouTube video² about pressure steaming with the Instant Pot. At the very start he states:
At 4:05 he uses the "steam" function. I'm not going to describe the video in detail because I suspect it echoes the manual for this particular process.
Note also about pressure steaming in the Instant Pot, the video states at 2:30 that a typical stove-top pressure cooker runs at 15 psi but the IP runs at 10.15-11.6 psi, and thus recommends increasing pressure cooking times for stove-top recipes by 7-15%. I can't personally comment on or confirm this as I don't have a lot of pressure cooking experience.
Regular Steaming How-To
As for normal steaming, I do not own an Instant Pot and can not say for sure, but I found this article about using it as a steamer (author is a different Jason), which pretty much hacks it with the "sauté" function. In particular (emphasis mine):
The article gives the following steps:
It also makes a note about the initial step:
The key point here is there appears to be an optional glass lid with a vent hole. If you don't have this accessory you might have a vented saucepan lid that fits. Also don't forget a steamer basket.
Now, I don't know if that applies exactly to your model (not sure which one the author of the article has) or if the linked products fit, so that part's up to you.
1 For an extra $29.99 you can even get the 9-in-1 "leak stopper" version which can catch rainwater if placed under a leaky roof.
2 Which perfectly complements my long-winded answer...