I may be the person who gives the advice more than anyone else. The reason I give this advice is many people who haven't traveled much don't factor travel time into their equations. When you watch a travel show, it's easy to think you can tour the leaning tower of Pisa in the morning and the Roman Coliseum in the afternoon, because the shows simply fade from one to the other. The reality is it takes time to relocate.
I also advise planning only two major sights a day, one in the morning one in the afternoon, with lunch in between. Can you see more sights in a day? Sure. Should you plan more than that? I'd say no. It's better to find yourself with extra time and add a sight than constantly try to keep up/catch up with your schedule.
Travel days are exactly the same. Might you be able to see some sights on a travel day? Certainly. IMO, however, it's better to start with the understanding that packing, checking out, eating breakfast, getting to the train station/airport (or getting out of town in a rental car), finding the right train/plane/car route, transit time, eating lunch, getting into town, finding parking or transportation from the train station/airport, checking into a new lodging, getting unpacked, getting oriented, etc. as well as the inevitable missed connections, delays, wrong turns will consume most of a day; each step consumes some time. Give yourself extra time for things to go other than perfectly without compounding the stress by having too tight of schedule.
I'd say consider any time for sight seeing on a travel day as serendipity rather than building an itinerary that relies on it.
As an example, I'm flying to Boise Friday to pack up my mom and bring her back to Virginia to live with me. I've moved her across several states before. She's a good packer but until I see the load, the van I'm renting, etc. I don't want to build a schedule that relies on me packing in three hours. I built another day into the schedule just as slack time in case everything isn't perfectly smooth. It might end up being a wasted day but the alternative may have been to plan an impossible schedule.