1)
Why did he capture the Maberos?
1.
Rashi: This was to be able to guard [that people not cross to escape].
2)
What is the meaning of "Efrati"?
1.
Radak: It is someone from Efrayim. Also, someone from Beis Lechem Efras, e.g. "v'David ben Ish Efrasi" (Shmuel 1:17:12).