From the definition, a collision would mean you would have to find any two arbitrary
y such that
h(x) = h(y) where
h is some cryptographic hash function. This is different from a second pre-image which means you have to find some
y such that given some specific
h(y) = h(x) (where
x != y). The key difference being that
x is fixed upfront.
I can make many cases why weak second pre-image resistance can hurt you (e.g. replace a known-to-be-good hash of a message/file with a malicious message/file that has the same hash), but I can't immediately come up with an attack where a collision would help you much. What are some real-world or theoretical attack vectors where abusing a weak collision resistant hash could cause harm?