Rashi: Just like I can change the seasons through my Tefilah, so if war would confront you, my Tefilah had power to stop the enemy. You did not need to request a king in my lifetime, even though I am old!

Radak citing his father: Rain never falls in Eretz Yisrael the entire time of the harvest - "ha'Nosen Geshem Yoreh u'Malkosh b'Ito Shevu'os Chukos Katzir Yishmar Lanu" (Yirmeyah 5:24). "MiTechilas Katzir Ad Nitach Mayim Aleihem" (Shmuel 2:21:10) - from Nisan 16 until Cheshvan 17 (Yerushalmi Kidushin 4:1). Since rain at the time of harvest was a big Chidush, they were afraid and asked Shmuel to pray for them. If rain comes after Nisan, it is a sign of a curse (Ta'anis 12b)!