Why is JAXA doing this?

The goal of the Small Carry-on Impactor experiment—SCI for short—is to learn more about how asteroids like Ryugu change over time. More specifically, JAXA scientists are hoping to learn:

How big a crater will form on Ryugu from an impactor of known mass and speed, by comparing before and after pictures at the crater site

What the materials beneath Ryugu's surface that haven't been exposed to space weathering look like

Whether or not the surface moves easily when struck by an impactor

How easily boulders shatter due to impacts, if the impactor happens to hit a boulder

There's a lot of uncertainty involved with SCI, which makes it a really interesting experiment. Scientists aren't quite sure how big the crater will be. They can only aim SCI to a certain degree; the target spot has a margin of error of 200 meters. The width of the crater will depend on the type of material the impactor hits. Most models estimate a crater up to 10 meters wide, but there are a few surface types that could result in either a very small crater (if the impactor hits a particularly porous spot) or a crater even larger than 10 meters (if it hits a bunch of tiny, coarse pebbles). The crater depth is expected to be about a tenth of its diameter, which is actually a rule of thumb for simple, bowl-shaped craters anywhere in the solar system!

The target area is near Ryugu's equator, about a quarter of the way around the asteroid east of the first sample collection site. That’s roughly 300 meters from MASCOT’s final resting place, so it’s not impossible that SCI could hit quite close to the now-dead lander. Since the Hayabusa2 team might also collect a sample from this artificial crater, they chose a spot geologically similar to the first touchdown site, allowing for an above-and-beneath-the-surface comparison. The location is also relatively free of hazards, which should help for a future touchdown.