What is the use of SCRAM-SHA-256 (over SCRAM-SHA-1)?
Nobody should use a primitive which is known to be broken in any new development or deployment. Anything involving SHA-1 is basically deprecated at this point.

A break in collision-resistance is a crack that might be exploited further by follow-on research; this happened with other hash functions in the past. Attacks only get better, they never get worse.

Besides, SCRAM-SHA-1 really isn’t deployed widely enough  for interoperability to matter.