Hayabusa and Hayabusa2 are sample return missions from asteroids. But little bits of asteroids fall to Earth all the time as meteorites. Why can't we just study those? What are the advantages of going to get a sample off the surface of an asteroid? What can we learn from collecting samples in situ that we can't guess from studying the stuff that falls down to Earth?
This can be answered in two ways.
At the Earth's orbit we do not necessarily sample (meteoroid intercepting Earth and surviving atmospheric entry to fall as a meteorite) all the taxonomic types (spectral classifications as presented by Bus et al., 2002, DeMeo et al., 2009, DeMeo et al., 2015) of asteroids in the form of meteorites. For example, more than 80% of the finds are ordinary chondrites (hence the name ordinary) and less than 5% of meteorite falls are carbonaceous chondrites.
In the context of Hayabusa2 and OSIRIS-REx missions which are concurrent now with former having returned its sample last December and the latter on its way back to Earth expected in September 2023, these samples become more interesting as they are expected to preserve the regolith (dust-like very fragile component present on the surface of these asteroids) which would otherwise be lost during the high-temperature atmospheric entry of a meteor. Therefore, these missions are returning material that is probably not yet present in our usual meteorite collection.
Check these videos to get a glimpse of what happened when OSIRIS-REx attempted the sampling. You may see the regolith, the material that is lost from the meteorites during atmospheric entry, in action.
What is of further interest is the fact that, Hayabusa2 has returned samples from the asteroid Ryugu which is a C-complex asteroid, whereas OSIRIS-REx is on the other hand is returning samples from the B-type (a subclass of the C-complex asteroids) asteroid Bennu). Both these asteroids correspond to the category of less than 5% meteorite falls as mentioned above in (1). It is believed (Altwegg et al., 2015) that these C-complex asteroids (parent bodies of carbonaceous chondrites) are responsible for bringing organic material and other chemical ingredients to the primitive prebiotic Earth to sparkle life. Therefore, these two missions have astrobiological importance as well.
The first Hayabusa mission, which you referred to, returned samples from the S-type (corresponding to the aforementioned ordinary chondrites) asteroid Itokawa in 2010, pioneering the era of sample return missions from asteroids.
But little bits of asteroids fall to Earth all the time as meteorites. Why can't we just study those?
Scientists do study those. Clair Cameron Patterson, for example, determined the age of the Earth between the late 1940s to mid 1950s using meteorites. However, he had to spend years addressing contamination issues before he developed his estimate of the age of the Earth.
Another example is the the collection of meteorites thought to be from Mars. Some of them (most famously, ALH84001) may have some signs of ancient life - life on Mars. The issue of potential contamination makes most scientists think the evidence is not conclusive.
What are the advantages of going to get a sample off the surface of an asteroid? What can we learn from collecting samples in situ that we can't guess from studying the stuff that falls down to Earth?
Meteorites pass through the atmosphere and then sit on the surface of the Earth before being discovered. That can change the chemistry of the meteorite and introduce cracks in the meteorite. Sitting on the surface can introduce Earth-based material into the meteorite through those cracks.
Avoiding these issues is the primary driver for sample return missions, whether from the Moon, Mars, or an asteroid. The collected materials will be pristine, unchanged by passage through the Earth's atmosphere and uncontaminated by earthly materials.
We could be waiting a very long time, if ever, for a particular asteroid to impact the Earth. Using probes to take sames from asteroids gives a sample from a particular asteroid within a quick time frame so it can be analyzed "sooner" rather than later, if ever.
Additionally, not all asteroids are the same. They can have different compositions. Taking samples "now" via probes can gives information we may not get by other means.