Bash: clarify “Positional parameters […] should be checked”

[SebastianSimon]

The Variables section in the Bash article has this bullet point:

• Positional parameters of the script should be checked, those of functions should not

Could this be clarified, please? “Checked” for what? In what regard, specifically, do script parameters differ from function parameters? Maybe include an example of what “check” you’re talking about.