The purpose of union with only one member inside the struct

I saw such union inside struct definition in pure c code in Linux kernel sources struct cma_multicast (it's not the only one place. Seems that it is some common practice):

struct cma_multicast {
    struct rdma_id_private *id_priv;
    union {
        struct ib_sa_multicast *ib;
    } multicast;
    struct list_head    list;
    void            *context;
    struct sockaddr_storage addr;
    struct kref     mcref;
};

But I can't figure out what is the purpose of union with only one member inside the struct? Why can't we just type struct ib_sa_multicast *ib; ?

I read this post but it has no usage explanation and has C++ specificity only.

UPD:
Posted example from Linux kernel instead of proprietary code.