I almost always make reservations. Anywhere good (by that I mean quality of food, not level of fanciness) is going to be full of locals who as a matter of practice make reservations, even at the local neighbourhood spot. Typically, when a restaurant deserves a table for someone, they reserve it for the rest of the evening, under the assumption that the diners will take their time (as opposed to in the US where they generally assume each reservation will take about 90 min or two hours and then they can turn over the table). For this reason, you might walk into a seemingly empty restaurant and yet be unable to get a table, or told that you can sit but must be out in, say, 45 minutes, because those tables are already booked. Exception is pizzerias as I think someone else mentioned - those often don’t take reservations.
I feel like making a reservation can only help and is pretty easy to do, so why not? If there’s no option to reserve online, I often email the concierge at my hotel ahead of my stay and ask them to make the reservations for me.