I have a particular property with (believe it or not) 9 parcels. This is highly unusual but is explained in that he lived next to a hypochondriac who would get on debt to a hospital and have to sell off a bit of land to pay them. He bought each parcel as it became available. A sort of pay-as-you-go process of assembling farmland. Only he never combined them into one larger parcel as many would have done. Perhaps he did not understand how it works. Maybe he didn’t care about the extra taxes. He had a good job.
Why does not everyone combine parcels? Lethargy, coupled with a lack of understanding how it works, is obviously at play much of the time. An object at rest tends to stay at rest. But there is another reason, a good one. If you ever want to sell off parcels, give them to children, etc. then you do not have to go through subdivsion. It is already subdivided. You can sell off any that you want, any time you want, and there is no extra red tape, no administrative fees, you just do it.
Let me go into this a bit deeper. Some towns still allow you to build multiple homes on a single parcel. Not many years ago virtually every one in our area allowed this. But that is changing. Towns recognize that this can cause problems, both for them and for the landowner. An example: Farmer Smith lets his son build a home on his land. It made sense as the two worked the land together, father and son. But Smith dies and leaves everything to all 3 children who never got along. Sonny Smith spent his own money to build his home but never owned the land under it. His siblings can screw him if they wish; he can't move the home, or, if he can, it might be prohibitively expensive. And the siblings are paying taxes on his home, so the unfairness is not just one-sided. Or maybe Sonny Smith decides farming is not for him and gets a job in another state thereby needing to sell his home. Or perhaps Farmer Smith gave a woodlot to his other son because he used wood to heat his home (off the farm). That woodlot is totally surrounded by his fields, but that'[s OK as Smith is there to make sure Sonny (who, remember, doesn’t get along with #2) does not stop #2 from crossing the farmland to get to it. Now Smith dies and #2 can't get to his woodlot without Sonny’s and his sister's permission. So #2 decides to sell it. But who is going to buy a landlocked piece that they can’t set foot on it without permission. And it should be legal permission so they can't rescind it if they decide they do not like the new owner or if he does something to irritate them.
Situations like this occur regularly and Towns are deciding they don't need to deal with these kind of arguments. To become an arbitrator in a family situation invites the family to divert their hate to them instead. And suppose the Smith heirs do not pay the taxes, and the Town has to foreclose and sell the property to the highest bidder. Now these issues are theirs to deal with. So they are slowly creating regulations to avoid these problems. Just as their tax structure varies, not all Towns deal with it in the same way either. As a buyer, you had best be aware of these things. They happen rarely, but they do happen. Keeping a variety of tax parcels avoids some of this and gives you flexibility if you want to divide the land in the future. That's assuming you want to divide it in the way it already has been done.
Suppose the Town decided that everyone needs a minimum of 5 acres to build a home now. If parcels were combined, you'd have to hack off 5 acre (or so - 5 acres is a common minimum in this area) pieces to give land to children or sell it to others. (The new owners of title may allow you to continue to use the land not needed for a home - but that's normally an informal arrangement between the two parties.) If you owned just 9 acres, you couldn't even do that. But if that tax parcel is even 1/2 acre, it can be build upon (assuming it is otherwise suitable for building) as that parcel was grandfathered in, locked in.
These are things that you need to think about when you have multiple parcels, landlocked land, or plans to subdivide. Be sure to consult the Town as I have been speaking in generalizations here and may or not be precisely right in any one instance.