Laura Summers, on why Pol Pot and Ieng Sary were the focus of the tribunal:

This was a “people’s tribunal”, and within the Vietnamese revolutionary tradition, people’s tribunals involved holding carefully organized public meetings which would help the masses to identify and collectively to take action against the worst oppressors (e.g. the richest landlords in the case of the 1954 tribunals in the old Democratic People’s Republic of Vietnam).  Once freed from the moral and practical grip of their principal oppressors, “the people” – under communist party guidance, to be sure – could accept alternative leadership and would certainly be more able of their own free will to adopt better ways forward.  The Revolutionary Council that came to power on 7 January 1979 needed to develop a mass base and wanted to focus attention on what was described as the “debts of blood” to the Khmer nation. Their Vietnamese allies had been struggling since early 1978 to present the Democratic Kampuchea state and Pol Pot’s Communist Party of Kampuchea as guilty of crimes against all of humanity, including “genocidal” attacks against Vietnamese civilian populations in its cross-border military assaults of that year.  It was a very complicated situation but, in brief, the tribunal was a way for Cambodians to open a dialogue on the violence suffered by Cambodians and to prepare the way for a more humane but still communist revolutionary leadership.

The policy was to hold only the very top leaders guilty of crimes against humanity - termed "genocide" as it turned out - while at the same time inviting and welcoming as many cadres or supporters of the Pol Pot party as possible to change sides and to support the new People's Republic. Pol Pot, the prime minister of Democratic Kampuchea as well as general secretary of the CPK, and Ieng Sary, foreign minister of DK and the third highest ranking person in the CPK standing committee, were put on trial because they were the two best known leaders of the failed regime. Everyone else was (quietly) though to be politically "redeemable".

Alex Hinton, on the stance of the international community towards the Khmer Rouge and the Heng Samrin government at the time of the tribunal:

The international community’s position has shifted dramatically, depending on the currents of history. At the time of the PRT, geopolitics was front and center with Russia, Vietnam and Soviet-bloc countries supporting the Heng Samrin regime and the US, China, Thailand, Europe and other powers opposing it. And, what is even more devastating, this latter block rearmed the Khmer Rouge (and later even gave them Cambodia's seat at the United Nations), who had been routed, and thereby laid the seeds for almost two decades of continued civil war. At this time, may Western diplomats wouldn’t even utter the word genocide (one euphemism was “the unfortunate events of the past”). While the PRT was about international legitimacy and defending the invasion, it was also about domestic legitimacy and providing Cambodians with a sense of redress after they had endured such suffering.

Elizabeth Becker, on the tribunal's intended audience:

The tribunal was meant to influence both the international community and Cambodia. This was the first attempt to describe what happened during the Khmer Rouge period using hitherto secret documents of Democratic Kampuchea, the Cambodian communist party and Tuol Sleng.

Summers, on what the tribunal achieved:   

I would say that the trial succeeded in opening the archives of a highly secretive regime. The organizers and jurists didn't assess these archives very thoroughly, or attempt to do so in what we might describe as a fair-minded manner, but such a move was politically urgent and important. Remember that most Cambodians knew very little about what was going on, outside of their own cooperative or ministry, and this was the first opportunity to share experiences and to learn - maybe - that a "deviationist" communist party and a dysfunctional revolutionary state apparatus were to blame. The tribunal probably succeeded in making the nationwide breadth and scale of the social disaster more apparent to significant numbers of Cambodians. I doubt it had any significant impact on popular support for the PRK, which was too visibly backed by a foreign army.