Visitors sometimes prefer to book an hotel outside the historic centre of Venice. It could be in far end of the centre (Castello, Giudecca), the Lido or Mestre (which is in the mainland).
The main reason is trying to spend less than staying close to the centre. Its true that hotels could be less expensive but you have to consider that youll be quite far from main attractions and youll risk to spend lots of time (and money) on travel.
Another reason is trying to avoid thousands of tourist making noise all the day, but some areas of Venice arent noisy as you can think and during the night all the city is very quiet.
Sometimes travelers decide to stay in Castello district in order to visit the Venice Biennale or in the Lido during the Venice Film Festival.
So if you want to feel a real venetian experience you have to look for an hotel in city centre, otherwise if it isn't your first trip, you don't care spending time on travel or you are here for business and want to stay close to the airport you can consider to stay in Mestre, in the Lido (where there are beaches) or in the main islands but far from the tourist areas.