What is the point of “grep -q”

The exit status of grep doesn’t necessarily indicate an error ; it indicates success or failure. grep defines success as matching 1 or more lines. Failure includes matching zero lines, or some other error that prevented matching from taking place in the first place.

-q is used when you don’t care about which lines matched, only that some lines matched.

if grep -q foo file.txt; then
    echo "file.txt contains foo"
else
    echo "file.txt does not contain foo"
fi