Reasons for dirt plots.
1. Idoc loot house - idocers will place them to loot a house that is falling close by
2. Keep/castle blockers - if the house is in a keep or castle spot people will place a plot down to block the placement so they can have a chance at placing themselves or sell to placer
3. PvPer - a lot of pvpers dont want to customize a house. They just drop a plot, put some bags on the floor and they are good to go.
4. bugged house - there are a lot of houses that for some reason havent dropped after the account has been turned off.
There are probably more reasons, but those are what i can think of.