What is the significance of this Tefilah?
Rashi: When Ya'akov saw how the P'lishtim would poke out Shimshon's eyes and how he would Daven for Siya'ata di'Shemaya to avenge what they had done, 1 he Davened that Hashem should answer his prayers.
Ramban: Shimshon, the last Shofet, was also the only one to fall into the hands of the enemy. When Ya'akov saw that the salvation of Shimshon came to an abrupt end, he prayed for the salvation of Hashem, which lasts forever, as opposed to that of the viper and the Shefifon (which are only temporary).
Targum Yonasan: When Ya'akov saw how Gid'on bar Yo'ash and Shimshon bar Mano'ach arose to save Yisrael from their enemies, he declared that it was not the salvation of these two men (whose salvation was only temporary) that he longed for, but for the salvation of Hashem (whose salvation is permanent). 2
Rashbam: He Davened for Dan's salvation when he fought against the enemy.
Hadar Zekenim: When Yakov saw the might of Shimshon, he said that it is not proper to aggrandize oneself about his might, Shimshon did so, and immediately he became thirsty and needed to beseech Hashem. Power and victory are in Hashem's hands.