I've tried both with advance booking and without.

I always used to book in advance, but then I found myself forced to move on when I really wanted to stay in a place for a day or 2 longer. So I did a couple of trips with very little booking. For instance in Vietnam, I booked one night in a hotel & then did everything as I went after that. The same for part of a trip to Cambodia & some parts of Thai trips.

This all works if there are a number of candidate hotels located close together. For instance, I had been around Halong bay & was returning to my previous Hanoi hotel, when the one just down the street offered a much better room at a low price. This also permits changes of itinerary at short notice.

The disadvantage is (for example) when turning up in a town and not finding any vacancies until the 6th hotel visited.

Of course, there are disadvantages to booking on line.
In one instance, I booked a hotel in Ayyuthaya & the lady had over booked & placed me in a hotel around the corner, with a rating 0.5 below that for the hotel I had booked. The worst hotel of my trip & needless to say, she got a terrible review from me.

My last few trips have been 100% booked in advance, although if I were doing longer trips away from peak season, I would probably do some parts without advance booking.