Because software can be very difficult and complex. We need the SDLC as a framework to guide the development to make it more systematic and efficient.
We will be able to tell how long it will take to complete the project, to test and deploy. Not only that, you'll have an easier time debugging and finding flaws in the software program or make enhancement to it.
As like any other engineering development processes, software should have its own standardized process to accomplish the following: -->Proper Coding which comes only from proper design -->Reduces rework in the code in case of enhancements required -->Easy to modify code