Why is it significant that pollen contains a multicellular male gametophyte instead of just male gametes?

The gametophyte stage which is multicellular is used to produce gametes to perform the reproduction of plant to produce zygotes.

Explanation:

In the reproduction procedure of plant generation, the multicellular stage of haploid or the gametophytes through the mitosis procedure, makes gametes which helps in producing zygote. And to produce multicellular sporophyte these zygotes develops itself.  So, it is evident that to produce gametes, gametophytes are essential in pollens of plant.  

the generally accepted parts of modern cell theory include: all known living things are made up of one or more cells. all living cells arise from pre-existing cells by division. the cell is the fundamental unit of structure and function in all living organisms.