That all depends upon your purpose. Time frame. Destination. Personal limitations.
Have I left out any other reasons?
It has nothing to do with one type of travel being preferable over the other.
Both making your reservations, planning all the details and exploring as a solo traveler, couple or group can fit the bill. You have all the freedom to make the decisions, and perhaps, live with the lousy hotel choice as all part of the adventure to write home about later.
Now my husband and I had an ideal trip - we had a terrific urban hotel, too - for seven days to Iceland that we arranged on our own down to daily tours thanks to using Icelandair as our air carrier. We established our own pace and the trip more than met our expectations. We'd return.
On the other hand, there are times when it is easier to leave the details to an experienced tour company and forget the hassle. Often a tour company will cut through the lines at a busy site, or eliminate an overhyped place not really worth visiting and subsitute it with a backroads stop that becomes a highlight. A tour guide native to the particular country makes for the best. I remember our Portuguese tour guide making a morning stop at a cafe for pingo - a coffee in Portugal - overlooking a castle town that was so pleasing visually that we all said that we could have stayed there forever, pooled our money to buy the castle and found perfection. The pingo was to die for, also, in case you are wondering.
Last winter I used OAT (Overseas Adventure Travel) to arrange a trip for a friend and me to travel the Route of the Maya in Central America simply because there was so much to see we wanted a complete tour. A tour company buys hotel rooms in bulk and often gets a better price than if you tried to do it on your own, too. In our case, two older women traveling might not have been wise either considering the difficulties getting from one archeological site to another. We would never have come upon the remote Maya villages on our own, and without a native speaker, we would not have interracted well.
Then again, touring in your own car or camper in the United States or Canada can be a cost saver depending where and how you stretch your dollar. One of the best trips my husband and I ever took was visiting all five Great Lakes. We followed city roads to mighty desolate routes in Ontario, Canada and the scenery was magnificient as were the folks we met along the way.
Rock collect and beach walks to lightouses were picture perfect. We had no final time to return home in mind either, which was ideal I believe.
Traveling to be with friends or relatives makes for built-in tour guides and you will really live like the locals. Years ago when I was in my mere twenties I spent part of a summer in Europe for the first time with a friend who was temporarily teaching in the Netherlands. We traveled all over by car and my eyes were opened to a wider world.
When I go over my travel plans for the future, I see that I have a combination of types in the works.