Mark apparently included this story to impress the reader with the supernatural element represented by the angel. The women would have said to one another, "Who rolled the stone away? It must have been someone very strong." When they entered the antechamber of the tomb, they would have thought, "Who is this young man (Gr. neaniskos)? He must be very unusual." He appeared as a youth, but his strength and his unusual dress indicated that he was an angel (cf. Mark 9:3). He terrified the women. [source][source][source]
"It may be suggested that the purpose of the angel"s presence at the tomb was to be the link between the actual event of the Resurrection and the women. Human eyes were not permitted to see the event of the Resurrection itself. But the angels as the constant witnesses of God"s action saw it. So the angel"s word to the women, "He is risen", Isaiah , as it were, the mirror in which men were allowed to see the reflection of this eschatological event." [1][source]