Acquisition is the status that says "This Program is the first time this lead ever came into this system" So that program gets the "credit" for creating a net new lead.
This is very important if you are using RCE or if you rely on the Program Analyzer or Program Performance reports.
Normally acquisition is set for a net new lead that comes in via a related form. But you can assign this during a list import. The acquisition only occurs for net new leads.
yes, you should assign this to imported lists like Trade Shows, etc.
I might change my answer depending on how you are using your external tools. For instance, when you import a list from Data.com, are you importing that for a general use, or for a specific email? Or do you assign Data.com leads to a "Data.com Acquisition Program"?