Hi there. A playpen is nice if you have space - if not, it is not essential. People use them as a safe area for the hamster to have out of cage time - not all rooms are hamster safe for free roaming. And free roaming isn't necessary. I often used to use the top of our bed for out of cage time with our last hamster -and if the cage is in the same room and your hamster is fairly tame, this could be an option. Although still needs you to be there in case they try and jump off! I used to put rolled up blankets round the edge which slowed up any attempts to climb off so I could pick the hamster up before he got too far - and it creates a kind of "barrier" that may keep them inside the space.
For cage cleaning, there are occasions when you do need to take the hamster out of the cage, and putting the hamster in a pet carrier is fine - depending on the hamster! And how long you need to sort the cage out.
Our last Syrian was quite happy just going to sleep in his pet carrier for some time. Especially when he had his hay house in it to chew on if he woke up.
Our current syrian just constantly tries to escape from his pet carrier, chewing at the holes on top and isn't happy. So it isn't an option for any length of time for him.
I personally don't like leaving the hamster in a playpen, if doing a cage clean, unless it is in the same room very nearby, so I can keep an eye on him. If it is in a different room then someone else can keep an eye on him.
The last time I had to do a substrate change and change his set up slightly (he is getting older so needed to lower everything) I left him in his play pen in another room, with someone keeping an eye on him, but unlike as usual, he didn't explore and play, he sat looking very concerned and huddled in a hide - because he knew I was doing something to his cage! He could hear/smell what was going on.
So I think it's talking about two separate things really. Firstly there are occasions where the hamster needs to be put somewhere else, while you do a partial cage clean. A pet carrier or another cage is fine for that. Spot cleaning while they are in the cage, can work for most hamsters, but not all. Our current hamster is better out of the cage while I'm spot cleaning. Our last hamster was less stressed if he could see what was going on and follow your hand round just checking the changes were acceptable!
Secondly is out of cage time. It is good for hamsters to have out of cage time - ideally every 2 to 3 days, but again depends on the hamster - some want to be out a lot, some are reluctant to leave their cage, especially if it's a nice big one. But even reluctant hamsters benefit from out of cage time.
Their behaviour is very different out of the cage than in it. In the cage, they are more territorial.
It isn't necessary to have a playpen though, if you don't have the space or have cats which can be an issue. What is more important then is that your hamster is quite hand tame and used to you, so they don't frantically do a runner or can't be picked up easily (if not tame).
If the cage is in a bedroom, you could keep the door closed and have the hamster out on top of the bed, with a few toys etc, for some bonding time and variety/change of scene.
It could be good to have an area where the cats don't go, and ideally not in the room with the hamster cage as just the smell may stress the hamster. There are plenty of owners who have a cat as well - but the cats need to be kept away from the hamster.
As you say, you'll be keeping your hammy in his own area, so that is fine. But he could do with an area he can have out of cage time as well.
I used to have a cardboard maze/castle thing I used on top of the bed sometimes
Not a playpen exactly but an area with a few sort of walls to explore in and to prevent a complete runner.