You are staying in Florence Italy for several days and plan to take a train to Pisa one morning where you will spend about a half day before continuing on to Lucca the same day. You plan to stay in Lucca for 1 night before returning to Florence the next day.
Double booking your Lucca hotel with your Florence hotel has some clear advantages and disadvantages.
Advantages include only having to bring a small day bag with you for your Side Trip as you can leave your other bags at your hotel in Florence.
You won't need to check out of your Florence hotel only to return the next day and check back in to another or the same hotel.
Disadvantages include paying for two hotel rooms for the same night. You will also have to carry around all our bags with you as your travel between locations. This means that for your time in Pisa, you may need to pay for bag storage somewhere as bags are not allowed in the leaning tower or the cathedral.
Just a note, if you do not want to double book your hotel for a night, you can accomplish everything without creating a Side Trip. You would simply create your locations as Florence-Pisa-Lucca-Florence. This would be a straightforward trip where you stay in Florence, check out of your hotel, journey to Pisa then on to Lucca for a night. The next day you would travel back to Florence where you would check in to your hotel.
Day & Side Trips as Locations (Creating Side Trips involves some additional steps which are outlined here)