Why did they beating?

Answer 1

Messiah was beaten by Roman "lictors". These were the bodyguards of Pilate who were there to hear Jesus / Yeshua declare Himself to be the King of the Jews in his presence.

He was not a Roman citizen, so it was legal for Pilate to have any accused person flogged in order to elicit a confession. But Messiah was without sin, so He remained mute. This enraged the lictors who hated Jews and yet one Jew had challenged their prowess and pride. They nearly killed Him for not saying a word. Typically, two lictors addressed the accused who was tied so as to wrap arms around a large rock; one had a whip (flagrum) of several strands with iron or bone attached; one had an iron bar on the other side. There was no limitation on Romans and people did sometimes die after being flogged in this manner. The bar made deep bruises which the flagrum tore open. Hence, matza today is white (sinless) with piercings (nails & thorns) and stripes (flogging); it is burned on both sides (bruises) to represent the affliction from both sides of Reality: the wrath of Man and God.

Jewish law had a maximum of 40 lashes. If the flogged person died after the 40, the one who whipped him could be found guilty of manslaughter since punishment was intended, not execution. Therefore, wisdom prevailed and 39 lashes was the norm. The Jewish whip was made of three straps of leather from three different species and 13 blows were landed to deliver a total of 39 stripes. This is per Edersheim, the Messianic (Christian) Jewish author from the late 1800's.