Methylcobalamin works everywhere in the body on EVERYTHING.

You cannot live without it. As far as the brain goes, it is part of a trio of B vits (B6 and folate) that are involved with making neurotransmitters by donating a methyl group to the chemistry.
It also is involved in fixing axons that have been damaged and remyelinating them. It is a cofactor with serotonin to make melatonin, the sleep hormone.

It is used by the bone marrow to make blood cells. It is involved with bone density in the skeleton. It is used by the muscles for energy production.

Because it moves by passive absorption(osmosis) into the central nervous system, having a high level of it in the blood makes it easier to get to the brain. So for specific healing jobs, it is best to have high levels of it. Some is stored in the liver for the future as well.