Yang Weining and Le Zhicheng are Chinese physicists who've been working on a top-secret project for the military. The project involves using high-powered radio waves to destroy enemy satellites and requires a remarkable degree of expertise among those working on it.
That is why Yang and Le have brought Ye Wenjie to the Red Coast base, where the project is being developed. Ye is a very talented astrophysicist, and her exceptional skills will come in useful.
Ye is also the right candidate for the project in that she can stay at the base for a prolonged period of time, as she has no family. As Le tells Yang, it wouldn't be possible to sequester an entire family at the base for any great length of time, so they need to have someone involved in the project without family responsibilities.
To be sure, Ye is not Yang's first choice for the job, not least because she's been accused of treason, potentially making her too hot to handle from a political perspective. But as the only two other suitable candidates wanted to remain at the May Seventh Cadre Schools, Yang has no choice but to accept Ye.
All in all, Ye is not a conventional choice to work on such a top-level security project. But as Yang readily concedes to Le, the entire project is unconventional.