As others have mentioned it’s basically the foundation of plant biology/genetics. There are entire labs of the top minds in plant genetics working on similar lines of this single plant. Papers/PhDs are published every day characterizing arabidopsis genes, how they interact with other genes, and then how it all changes under different natural or artificial conditions. Because many plant genes are conserved across species, this translates to insights (or gives a good starting point) for researchers working on other plants (or more applied areas like crop improvement).

It is often a model organism in plant studies, similar to mice in drug discovery. Mostly because it has a quick growth time, is fairly hardy, and just the fact it has been so well studied means that a lot is known about it.

I am not a plant biologist but have talked to some and I've often seen it used in their work and this was their given reasons.

Is a famous organism in genetics. It grows and blossoms really fast and is well known so you can watch a lot of generations in less time than other plants. Is also conveniently small, so you can culture a lot of specimens in your valuable laboratory space. Is also a Brassicaceae, that are VIP (Very Important Plants) in our agriculture.

Wikipedia says its small genome and short "cycle time" makes it an excellent test subject for various experiments. A photo depicts it growing in sidewalk cracks.

Maybe they'll pick an orchid next time!