Typically, liners are used to deal with corrosion, and more generally, material incompatibility between the fluid and vessel wall.
316SS, duplex, hastelloy, etc. are materials. These are used, again, to deal with fluid to vessel wall material compatibility (eg. corrosion).
I say compatibility, and give corrosion as an example. Another examples may be reactions - the fluid may react with the vessal wall material to give a undesireable intermediate or product.
In the end, it all comes down to chemistry, and cost - mostly cost. Is the cost of exotic metal more economical than liner? Or, is a less ideal material more economical than the ideal material? How long does the tank have to last? What are the consequences of one material/liner than another/nothing?
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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