Jesus appointed these disciples for a twofold purpose: to be with Him, and to preach. The order is significant. [source][source][source]
"Fellowship with Him must precede preaching about Him." [1][source]
Jesus also gave these disciples the ability to cast out demons along with preaching. The miracles would convince many of their hearers that God had sent them as His spokesmen. Mark probably mentioned exorcisms because this was the greatest demonstration of the disciples" authority, not the only one. This Gospel documents Jesus" training of the Twelve in these two basic areas particularly: being with Jesus and preaching. [source][source][source]