As noted, it would be rare that a hotel doesn't let a guest stay in the same room, provided all of the consecutive reservations are the same room type. As someone who worked in hotels for over a decade, I can confidently say that making a guest change rooms is a big hassle for both the guest and the hotel.

For the hotel, there's a whole lot of labor costs involved in changing rooms. The Front Desk representative must coordinate the move with the guest. More importantly, check-out rooms get a more thorough cleaning (and new linens/towels) when compared to a stay-over room. So, it's actually in the interest of the hotel to keep the guest in the same room.

That said, it does happen occasionally. The reason is what Brighton Line noted: blocked rooms. Rooms can get blocked in the future for lots of reasons, including connecting rooms.

Follow the advice given and proactively mention the multiple reservations at check-in. That should be good enough unless you know you're traveling at a time of exceptionally high demand or checking in very late (after midnight). In those situations, a call to the hotel the day prior to your arrival would be smart so that they can make sure you get placed in a room that isn't blocked later in your stay.