Good advice here from Fio.
The biggest problem with not booking in advance is that you waste time looking for a place to stay when you could be out doing what you came to do.... see the place.

We recently had an interesting experience in Ho Chi Minh City which illustrates the issues pretty well............
After spending a few days at the beach, we returned to HCMC and arrived at the hotel we'd booked only to find that our room was on the 5th floor and there was no lift!
It was in a good location and I guess for 55 USD it was otherwise ok,but climbing up and down 5 big flights of stairs made it a no-go.
It was late afternoon and the travel agent I rang said that there would be limited availability that night except for a couple of budget options which we didn't want (same issue with no lift). I think also that because we'd booked by phone, they'd given the better rooms to those who'd paid a deposit or arrived before us, leaving us with the big climb!

My husband went to a hotel a few doors away and yes, they had a room which he saw and said it was ok so he booked us in at 105USD for the night..
On the way to the room I had a bad feeling (the hotel was tired looking and definitely needed a total renovation). It looked like it would struggle to get a 3 star rating from anyone.

We had a basic room with an uninspiring view of the windows of the hotel next door. Clean was about the best thing I could say about it, but certainly no frills except cable tv (and a lift!!) but I was glad we were going out for the evening and wouldn't be spending much waking time there.
Next day back at home, I checked this hotel on online booking sites - standard 49USD, superior (50USD)and deluxe (60USD) suite 117USD. No photos of standard but all the other room photos looked entirely different to what we had so I guess we had standard.
So walking in and paying rack rates is a hazardous thing to do and just confirms my belief that you put yourself in the position of paying way too much when you just turn up to a 3+ star hotel.