From Joel Marcus’s commentary on Mark 3:14-15 — “He appointed twelve that they might be with him and that he might send them out to preach and to have authority to drive out demons.”
But the Twelve are not only summoned to perform acts of proclamation and exorcism; those acts flow out of a prior commission, the call to “be with Jesus.” This tension between being with Jesus and being sent out by him is most simply resolved by interpreting 3:14 and 3:15 sequentially: now the disciples are with Jesus, but later he will send them out to preach and exorcise (cf. 6:7, 12-13). But Mark’s odd formulation probably also contains another layer of meaning. Throughout the Gospel, Mark speaks of the disciples being with Jesus or his being with them (1:29; 2:19; 3:7; 4:36; 5:37, 40; 6:50; 8:10; 9:8; 11:11; 14:7, 14, 17, 18, 20, 33, 67) . . . In other cases Mark seems to have introduced references to the Twelve or to the disciples generally (e.g. 2:15; 3:20; 6:1; 11:11) or to have highlighted their presence rhetorically by his use of plurals (e.g. 11:15, 19, 27). These features have the effect of portraying Jesus as one who is almost constantly surrounded by a circle of disciples; he does not exist primarily as a solitary individual but as a being-in-community, and living the Christian life means “being with him.” . . . In this light there is another way of reconciling the tension within 3:14: now, in the post-Easter period, it is possible both to be with Jesus and to be sent out by him; Mark, in fact, would probably say that any mission not rooted in “being with Jesus” is doomed to failure (p. 267).