I would never go to Europe without my computer. I first started taking one in 2000, when I went with my first digital camera, and memory sticks cost a fortune for one big enough for only a few pictures, so I took my laptop and downloaded my pictures every night to free up the memory stick for the next day. This, remember, was before everyone had a cell phone, before there was a cloud, and when there were few places with WiFi to upload your pictures.
I found so many uses for the computer on that trip that I would never think of not having it with me. Since then I have owned a series of laptops, handhelds, and netbooks just for traveling. (At home I have big desktop with a real keyboard and 32" monitor; on the road I have a 2# netbook with a 10" screen and a real keyboard.)
During that first trip, because I had a computer available, I created a spreadsheet version of the expense report companies had made me fill out when I traveled, and found it a very useful tool for understanding where my money went, and therefore to plan for future trips. I also used my computer to write a log of my travel (later, when WiFi was more available, I would upload my trip report to my website for others to read). For the next trip, I found many more uses for the computer. I created a giant "workbook", with pages for every day of travel, with rail schedules, my preferred connections and alternatives, and with pages for places I would visit, with downloaded maps and information on attractions.
I created a dummy expense report with all my known and expected expenses. As I paid for things, I took them off the spreadsheet; that way at any time, I knew pretty closely how much I needed to take out from ATMs to finish my trip.
Of course, I could, with difficulty, do all this on a phone today, but, as someone else pointed out, it's difficult to see a spreadsheet on a small screen. I learned to touch type in HS, and I find the phony keyboards on phones to be very aggravating because they are so inefficient. I can touch type much, much faster than I can pick keys on those tiny keyboards, and, with a real keyboard, I don't have to look. I find it so irritating, on a phone keyboard, to be constantly hitting the wrong, tiny key and having to stop and backspace.
But the real shortcoming of a phone computer was brought home to me last summer when my partner and I visited her relatives in Maine. They had just moved into a house in a rural area and they were trying to get the best route into town. Siri was of absolutely no help (maybe she didn't know all of the minor roads, I don't know, but she was taking them on a long, roundabout way into town. I had just acquired my first "smart" phone (my old flip phone was G3, and they weren't supporting it any longer). On my phone's screen, if I could see the entire route into town, I couldn't see the names of the actual roads, and if I zoomed in enough to see the road names, I couldn't see the big picture. So I fired up my netbook, which fortunately I had brought along, and, on the bigger screen could see the whole route.
To me, phones are just a poor substitute for a computer, if you can't access one, but they are always a poor substitute.