Background: Pakistani Prime minister Imran Khan was the first foreign leader to visit Russia after beginning of the invasion.
Usually such actions (as a public letter, petition, etc.) are undertaken to put the public pressure on a government and/or discredit the actions of this government in the eyes of the world community and general public.
Pakistan is one of many countries who have strong economic, political and/or military ties with Russia, Ukraine and the countries members of NATO. Obviously both sides are working hard to push these countries from sitting on the fence into unambiguously supporting the preferred cause. Pakistan is either judged to be country requiring just a little push... or it is judged to be leaning towards Russia and the letter might be the last ditch attempt to keep it neutral. Pakistan might be particularly important in connection to India decision to remain neutral, so Pakistan-India rivalry could be exploited to put more pressure on India.