cp $@ ~/dir

cp "$@" ~/dir

Double quotes around $@ (and similarly, ${array[@]}) prevents globbing and word splitting of individual elements, while still expanding to multiple separate arguments.

Let's say you have four arguments: baz, foo bar, * and /*/*/*/*

"$@" will expand into exactly that: baz, foo bar, * and /*/*/*/*

$@ will expand into multiple other arguments: baz, foo, bar, file.txt, otherfile.jpg, and (eventually) a list of most files on the system

Since the latter is rarely expected or desired, ShellCheck warns about it.

When you want globbing of individual elements.