[One of the sights upon which one makes the beracha for miracles is] the rock on which Moshe sat [raising his hands, as Yehoshua fought the Amalekis], as the pasuk says: "And Moshe’s arms were heavy, so they took a stone and put it under him, and he sat upon it, and Aharon and Chur supported his arms" (Shemot 17:12). Ein Ayah:
It was not possible for Moshe himself to wage battle against Amalek. Amalek is the arch-enemy and spiritual opposition of Israel. Moshe, the father of the prophets, who viewed Jewish history to the end of the generations, saw that there was still a need for Amalek. Until Israel purifies itself, there is still a need for the opposing force that Amalek engenders. That is why Moshe’s arms were heavy, and he had to let them down.
Aharon and Chur were able to have the proper outlook on the generation and its needs at the time, and they showed their power by supporting Moshe’s hands. Moshe did not stand, which would display his strength, but lowered his stature by sitting, taking into account the needs of the generation, which definitely included subduing Amalek, if not destroy them outright.
Due to the above, Yehoshua did not destroy Amalek but only weakened them, as the pasuk says (Shemot 17:14). Therefore, the stone is an eternal remembrance of Hashem’s kindness to His nation, to make their enemies succumb, as was the need of the time, even if "eternity" does not fully agree with it. Opposing Evil
(condensed from Ein Ayah, Berachot 9:12) Gemara:
All the miracles upon which we make a blessing are understandable except for the pillar of salt that Lot’s wife turned into, which was a sad event. On Lot and his wife one makes two berachot: "Blessed is … the judge of truth" on Lot’s wife and "blessed is He who remembers the righteous" on Lot. Ein Ayah:
The foundation of the destruction of Sodom was the great value of kindness and the damage from the loss of kindness in the world. Avraham, the great kindness provider, was the anti-thesis of Sodom. When there are philosophical disputes in the world, there are often those who oppose an idea, but only in the negative. In other words, they are bothered by the problems with a certain viewpoint. However, until they take the initiative to build an alternative, which they use to lead the world in a more correct manner, they have not completed that which is desirable.
Sodom had an organized social doctrine that was based on the hatred of kindness. It was not enough for others to disagree with them, but they should have developed a system based on kindness. This is what Avraham did, as the pasuk says: "… that he commands his children and household after him that they should observe the path of Hashem to do charity and judgment" (Bereishit 18:19).
That is why one should connect the downfall of Lot’s wife, which shows the hatred of systems that oppose kindness, together with the salvation of Lot, who was saved because of his connection to Avraham. Remembrances can relate to opposites. When one thinks of cold, he remembers heat; when he remembers white, he thinks of black as well. So too, when one remembers Sodom, he thinks of Avraham.
The educational idea is that one should fight the bad not just with dormant opposition but by building the antithesis of evil, along the lines of Avraham. This is a general rule regarding any approach that negates truth and justice, as Avraham taught.