Why does the Instant Pot use pressure while steaming?
TL;DR


The Instant Pot is primarily a pressure cooker and its "steam" function is for pressure steaming. I guess typically you do raw/frozen veggies and seafood with this function.
You can do normal steaming in "sauté" mode with a vented glass lid (sold as an optional accessory, or if you have a vented lid that fits).




Why?


  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? 
  
  ... my question is about why the IP uses pressure for steaming, not how to steam with an IP. The technique described in the link doesn't use the steam function of the IP, it uses the sauté function. 


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:


  The steam function is mostly used to cook raw or frozen vegetables as well as shellfish and other seafood.


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):


  I’m not talking about pressure steaming here, but simply using the Instant Pot as a countertop steamer.
  
  ...
  
  The optional Instant Pot glass lid has a small vent hole at the top that releases some steam. The power of the steam in the pot also causes the lid to rattle ever so slightly, the same way some slow cookers do.


The article gives the following steps:


  
  Place your steamer basket and 2 cups of water into your Instant Pot
  Set Instant Pot to saute mode, and adjust the heat level to ‘more’
  Place the glass lid onto your Instant Pot to allow the water to reach a low boil / simmer
  Add your vegetables to the steamer basket and re-cover the pot with your lid.
  Steam to desired doneness!
  


It also makes a note about the initial step:


  The water reached 203°F after 6 minutes without the lid on. The water will reach temperature a minute or so faster if you leave the lid on while preheating.


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...