The term bog is normally in the section, “description of tenements” which is used to describe each property. So by listing it as bog, it will explain why the value is lower than for adjacent land, capable of growing crops etc. The possession or lease of a piece of bog doesn’t grant any automatic right to cut peat (turf). That is covered by separate rights, usually called turbery, which are often mentioned in leases but not in Griffiths. The right to cut peat doesn’t directly affect the value of the land, and so wasn’t of interest to Griffiths.
A person with a piece of bog would probably use it to graze a few sheep or cattle. It may also be used for turf cutting but, as I say, that would be specified separately elsewhere. Note that if the bog is described just as land, then normally that means the owner/lessee didn’t have a house there, and must have lived elsewhere.