It looks like some sort of expansion tank. Do you have a pic from further out or a different angle so we can see what it's attached to?
Not sure if it's water or air given that it's stainless, unless salt or potable water. That's why I was hoping for a wider pic to see what it's connected to.
My saltwater washdown has a stainless tank like that. Don't know why they are all not stainless. They are not more money then the painted ones
That would make sense for it. Of course stainless is more expensive. Even if it's pennies though no sense paying for what's not needed.
WOW, I must have been using the wrong glasses when I looked at this the first time. Yes clearly and accumulator / pressure tank for something.
It is a Shurflo tank; Shurflo, Stainless Steel Accumulator Tank 2 Gallon The tank can be used for a fresh water system, in a chilled water A.C. system, or a salt water system, as examples. As NYCAP says, a broader view may reveal purpose.
I think the other guys are correct as a expansion/ Accumulator tank with a nice bracket on top to hold the water pressure pump