1. Estimates (i.e. budget, schedule, etc.) become more realistic as work progresses, because important issues are discovered earlier. 2. It is more able to cope with the (nearly inevitable) changes that software development generally entails. 3. Software engineers (who can get restless with protracted design processes) can get their hands in and start working on a project earlier.
Disadvantages of Spiral Model
1. Highly customized limiting re-usability 2. Applied differently for each application 3. Risk of not meeting budget or schedule 4. Risk of not meeting budget or schedule